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Article

Impact of Terrigenous Organic Matter Input on Organic Matter Enrichment of Paleocene Source Rocks, Lishui Sag, East China Sea

by
Xu Han
1,2,
Dujie Hou
1,2,*,
Xiong Cheng
1,2 and
Yan Li
1,2
1
School of Energy Resources, China University of Geosciences, Beijing 100083, China
2
Key Laboratory of Marine Reservoir Evolution and Hydrocarbon Accumulation Mechanism, Ministry of Education, China University of Geosciences, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Energies 2023, 16(4), 2046; https://doi.org/10.3390/en16042046
Submission received: 16 January 2023 / Revised: 11 February 2023 / Accepted: 17 February 2023 / Published: 19 February 2023
(This article belongs to the Special Issue Advances in Petroleum Geology and Unconventional Oil and Gas)
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Abstract

:
To clarify the organic matter (OM) enrichment of the Lishui Sag, the factors influencing the variable abundance of OM in the Lingfeng Formation are studied using organic geochemical data. The source rocks of the Lingfeng Formation have medium–high total organic carbon (TOC) values (0.53–3.56%). The main type of kerogen is II2-III. Compared to the shallow marine subfacies source rocks, the TOC of the delta front subfacies source rocks is higher. The distribution of biomarkers shows that the redox environment of the delta front subfacies source rock is the sub-oxidizing and oxic environment, and the source rock is mainly supplied by terrigenous higher plants; the redox environment of shallow marine subfacies source rocks is a sub-reducing and suboxic environment, and the OM mainly comes from algae. The link between OM input and OM abundance demonstrates that terrigenous OM (TOM) input has a considerable influence on OM abundance. However, there is no obvious relationship between preservation and OM abundance, which suggests that preservation is not the determining element in OM enrichment. The strong sediment flux decreases the amount of time that OM is exposed to oxygen. As a result, delta front subfacies with large TOM input have a huge number of excellent source rocks. This paper proposes a “delta front-OM input model” for excellent source rocks.

1. Introduction

The primary prerequisite for the accumulation of hydrocarbons is an abundance of organic materials in the source rocks [1]. Excellent source rocks contribute 80% of oil and gas and control the advantageous transport direction of oil and gas [2,3]. Therefore, determining the organic matter (OM) enrichment mechanism in source rock is an important part of oil and gas exploration and development [4,5,6,7,8]. Over time, a new topic of study has emerged: the enrichment of organic content in marine source rocks. Many successful studies on the organic material enrichment mechanisms of marine source rocks have been carried out [9,10,11]. OM supply, redox conditions, paleo-salinity fluctuations and paleoclimate control the OM enrichment of marine source rock [12]. OM supply includes primary productivity at the surface of the ocean (plankton and algae) and terrestrial OM input [4,5,13,14,15]. OM supply is the basis for OM enrichment. Redox conditions have a strong influence on OM preservation [12,16,17]. Reduction conditions are conducive to OM preservation [10,18]. High paleo-salinity is beneficial to OM preservation. The warm and humid paleoclimate favors biological growth [10,18,19]. Large rivers have a strong influence on OM enrichment. Aquatic organisms thrive due to the nutrients brought by rivers [13,20]. Meanwhile, the amount of OM in the water is large because of the terrigenous organic matter (TOM) brought by rivers [20]. However, oxygen is carried by large rivers into sedimentary waters, causing the sedimentary environment to become an oxidizing environment, which is unfavorable to OM preservation [1,20].
Six oil-bearing structures and one petroleum and natural gas field have been discovered in the study area since the 1990s [21]. The Lishui Sag has become a new focus of exploration. Hydrocarbon accumulation factors in the Lishui Depression have been researched by many scholars. Previous studies have clarified that the Lingfeng Formation (E1l) and the Yueguifeng Formation (E1y) are the primary source rocks, and the petroleum and natural gas are mainly derived from the E1l [21,22,23]. Trap formation time and passage opening time are all related to the oil and gas charging period [24,25,26]. Maturation is conducive to sand body development in the gently sloping zone of the western Lishui Sag, and this area is favorable for oil and gas charging and preservation [27,28,29,30]. The OM enrichment modes of the Lingfeng and Yueguifeng Formations have also been studied [21,22,31]. However, previous works have mainly focused on the differences between the Lingfeng and Yueguifeng Formations. The Lishui Sag has an excellent drainage system and reservoir sealing conditions. However, during the depositional period of the E1l, the sedimentation rate of the sedimentary facies was rapid [22,24,30]. The location of excellent source rock development changed rapidly. This change led to the differential accumulation of hydrocarbon. There is a paucity of systematic research on the modes of OM enrichment, the primary factors governing the development of excellent source rocks, and the differential accumulation in a single formation. The ambiguous OM enrichment mode of a single interval severely restricts the process of exploration. Therefore, this paper uses E1l source rocks as the research object and employs organic geochemical analyses to establish new source rock evaluation standards that are suitable for the study area. Based on the planar distribution characteristics of excellent source rocks and combined with macroscopic carbon isotope and microscopic biomarker compound analyses, the mechanism of OM enrichment is analyzed. The main factors influencing the development of excellent source rocks are investigated, and finally, an OM enrichment model is established. The location of excellent source rock development is indicated, and theoretical guidance for the next phase of exploration in the Lishui Sag is provided.

2. Geological Setting and Stratigraphy

The Lishui Sag is located in the western East China Sea Shelf Basin (ECSSB) [32,33]. The ECSSB lies at the convergence zone of the Eurasian, Pacific, and Philippine plates. Two uplifts split the Lishui Sag to the west and south (Figure 1) [26,34,35]. The morphology of the Lishui is controlled by the NE-trending segmental faults. The Lingfeng low bulge divides the study region into two parts: western and eastern [21,26,27]. The tectonic evolution phases of the Lishui Sag from the Cretaceous to the Tertiary are impacted by variations in the western Pacific plate’s subduction direction and rifting. The tectonic evolution phases of the study area includes a rift stage, a post-rift depression stage, an uplift stage and a regional subsidence stage [23,26,27,29,31].
The stratigraphic column of the study area includes Late Cretaceous, Paleogene, Neogene and Quaternary strata. By integrating biostratigraphic data, conventional core facies, gamma ray (GR) logging facies and seismic facies characteristics, a three-level sequence stratigraphic framework, including six key horizons, namely, T80, T83, T85, T88, T90 and T100, was constructed (Figure 2) [21,28,30]. The primary source rock is the lower Paleocene Yueguifeng Formation (E1y), with the overlying Lingfeng Formation serving as a secondary source rock (E1l) [21,22,26].
During the depositional period of the E1l, three main sedimentary facies developed: fan-delta, delta and shallow marine facies (Figure 2) [21,22,30,31]. The delta plain subfacies are widely developed in the western slope belt of the west subsag, and the delta front subfacies are locally developed in the steep slope belt. The east subsag is controlled by the provenance on both sides of the low Lingfeng Uplift and the Yandang Uplift, and fan-delta facies are widely developed in the eastern part [21,30]. The source rocks of the Lingfeng Formation can be divided into delta front subfacies (wells A, C, E and F) and shallow marine subfacies source rock (wells B, D and G).
The lithology of E1l is dominated by gray, dark gray and black mudstones with interbedded thin-layered, light-gray calcareous siltstones, fine sandstones and a few thin-layered calcareous fine sandstones [21,30]. The sandstone gradually changes from fine sand to silt in the upper part, and the calcium content of mudstone increases, forming a positive cycle of coarse grains at the bottom and fine grains at the top [24,25,31]. The calcium content of mudstone decreases downward, and the sandy limestone that contains algae and other bioclasts has an uneven thickness but is distributed in the whole area and can be used as a marker layer. The Shanghai facies mudstone can be continuously tracked in the area and is not at an angle to the underlying strata [21,26,27,28].

3. Materials and Methods

A total of 119 samples of gray to dark gray delta front–neritic mudstone from the Neo-E1l were collected from seven exploratory wells, namely, A, B, C, D, E, F, and G, in this study. Among them, 76 pieces were samples of rock cuttings, and 43 pieces were samples of rock cores. In this paper, total organic carbon (TOC) analysis, pyrolysis, and biomarker compound analysis were carried out at the China University of Geosciences, Beijing. Before the start of the experiment, 126 samples were cleaned to remove drilling fluid. Then, the best samples were selected for testing.
A total of 119 samples were analyzed for TOC analysis and Rock-Eval pyrolysis. The source rock samples were crushed to 80 mesh. The inorganic carbon was removed using dilute hydrochloric acid. Then, the source rock samples were analyzed using a CS-580A C/S analyzer to estimate the TOC according the GB/T 19145–2003 standard. The samples utilized for the TOC analysis and pyrolysis analysis were processed in the same way. The measured parameters included the TOC, volatile hydrocarbon HC content (S1), remaining HC generative potential (S2), and maximum pyrolysis yield temperature (Tmax). The hydrogen index (HI) was calculated from the TOC and S2. The parameters were determined using the GB/T 18602-2012 standard as a guide.
A total of 43 rock core samples were used for chloroform extraction and group component separation. All samples were crushed to 80–120 mesh, and 100 g of the samples was placed in a 75 °C water bath for Sox extraction for 72 h. The extracts were recovered by an evaporator to recover chloroform and evaporated to dryness for later use. In accordance with the SY/T5119-2016 standard, the extracts were separated on silica/alumina gel and eluted with n-hexane.
A total of 43 samples of saturated hydrocarbon components were analyzed by gas chromatography–mass spectrometry (Agilent 7890-5975c) with a DB-5MS elastic quartz capillary column (60 m × 0.25 mm × 0.25 μm). The velocity of carrier gas was held constant at 1 mL/min. The carrier gas was 99.999% helium gas, and the temperature at the inlet was 310 °C. The temperature was held constant at 50 °C for 4 min throughout the heating program, and then it was raised to 300 °C at a rate of 4 °C/min and held there for 35 min. The parameters were determined using the GB/T 18606-2001 standard as a guide.

4. Results

4.1. Abundance of Organic Matter

The values of TOC in the E1l are between 0.53 and 3.56%, with an average of 1.16%. The HI values of the source rocks are between 8.14 and 204.59 mg HC/g TOC, with an average of 90.7 mg HC/g TOC. The hydrocarbon generation potential (S1 + S2) values of the source rocks are between 0.07 and 3.59 mg/g, with an average of 1.41 mg/g (Table S1). In general, the source rocks of the Lingfeng Formation have good potential for producing hydrocarbons.

4.2. Type of Organic Matter

According to the van Krevelen plot of hydrogen index (S2/TOC X100) and Tmax displayed in Figure 3, the type of OM has been categorized. The figure shows that the kerogen type of the source rock of the E1l is dominated by types II2–III [36]. Kerogen types II–III suggest that the source rocks of the E1l have different abilities to produce petroleum and natural gas [37,38,39,40]. The source rocks of the E1l are mainly gas-generating, and a small number are oil-generating [41,42,43,44].

4.3. Molecular Biomarkers

4.3.1. Normal Alkanes and Isoprenoids

The distribution characteristics of n-alkanes in all samples can be divided into three types (Figure 4). The chromatograms of saturated hydrocarbon from the samples of some delta front subfacies source rocks (wells A and F) show a unimodal distribution of alkanes between C14 and C35, with an excess of long-chain n-alkanes between C27 and C33 (Figure 4). The chromatograms of saturated hydrocarbon from the samples of the other delta front subfacies source rock (wells C and E) show a bimodal distribution of alkanes between C14 and C35, with an excess of short- and medium-chain n-alkanes between C17 and C22 and an excess of long-chain n-alkanes between C24 and C28 (Figure 4). The chromatograms of saturated hydrocarbon from the samples of the shallow marine subfacies source rock (wells B, D and G) show a unimodal distribution of alkanes between C14 and C33, with an excess of short- and medium-chain n-alkanes between C17 and C22 (Figure 4). The pristane/phytane ratio (Pr/Ph) of the delta front subfacies source rocks range from 0.63 to 4.81, with an average of 2.8. The Pr/Ph of the shallow marine subfacies source rocks are between 1.12 and 3.57, with an average of 2.7 (Table 1). The nC15+17+19/nC27+29+31 ratios of the delta front subfacies source rocks range from 0.13 to 5.91, with an average of 0.9 (Table 1). The nC15+17+19/nC27+29+31 ratios of the shallow marine subfacies source rocks are 0.59–2.57, with an average of 1.5 (Table 1). The nC27+29/nC31+33 ratios of the delta front sub-facies source rocks are between 1.15 and 2.44, with an average of 1.8 (Table 1). The nC27+29/nC31+33 ratios of the shallow marine subfacies source rocks are 2.09–3.56, with an average of 2.9 (Table 1). The average chain length (ACL) values of the delta front subfacies source rocks are 24.58–28.35, with an average of 27.2 (Table 1). The ACL values of the shallow marine subfacies source rocks are 25.49–26.75, with an average of 26.2 (Table 1). The aquatic microphyte (Paq) values of the delta front subfacies source rocks are between 0.27 and 0.95, with an average of 0.5 (Table 1). The Paq of the shallow marine subfacies source rocks range from 0.56 to 0.81, with an average of 0.7 (Table 1). The ratio parameters of the biomarker compounds are shown in Table 1.

4.3.2. Terpanes

As shown in Table 1 and Figure 5, the predominance of C30 hopane is displayed on the m/z 191 chromatograms. The source rock m/z 191 chromatograms of the Lingfeng For-mation show the development of C19 C20 C23 tricyclic terpanes (TT) and C24 tetracyclic terpane (TeT). The C19 TT/C23 TT C20 TT/C23 TT C24 TeT/C23 TT ratios were calculated. The C24 TeT/C23 TT of the delta front subfacies samples are 0.83–1.51, with an average of 1.31 (Table 1). The C24 TeT/C23 TT of the shallow marine subfacies samples are 0.16–1.51, with an average of 0.9 (Table 1). The C19 TT/C23 TT values of the delta front subfacies samples are 0.08–1.09, with an average of 0.5 (Table 1). The C19 TT/C23 TT values of the shallow marine subfacies samples are 0.11–1.20, with an average of 0.7 (Table 1). The gammacerane index (gammacerane/αβ-C30 ho-pane) values of the delta front subfacies source rocks are 0.06–0.22, with an average of 0.1 (Table 1). The GI values of the shallow marine subfacies source rocks are between 0.03 and 0.21, with an average of 0.1 (Table 1). The C35/C34 H(C35 22 S/C34 22 S hopane) ratios of the delta front subfacies source rocks are between 0.00 and 0.63, with an average of 0.3 (Table 1). The C35/C34 H ratios of the shallow marine subfacies source rocks are between 0.15 and 0.7, with an average of 0.5 (Table 1). The oleanane index (oleanane/αβ-C30 hopane) values of the delta front subfacies source rocks are 0.05–0.41, with an average of 0.1. The oleanane index (oleanane/αβ-C30 hopane) values of the shallow marine subfacies source rocks are between 0.01 and 0.15, with an average of 0.07. The parameters of the bi-omarker compound ratios are shown in Table 1.

4.3.3. Steranes

Considerable steranes, diasteranes and pregnanes were identified in the m/z = 217 mass chromatogram of all samples (Figure 6). The distribution of regular steranes in some delta front subfacies source rock (wells A and F) is dominated by an inverted “L” shape. The distribution of regular steranes in the other delta front subfacies source rock (wells C and E) is dominated by an inverted “V” shape. The distribution of regular steranes in the shallow marine subfacies source rocks (wells B, D and G) is dominated by an “L” shape. The C27/C27–29 S ratios of the delta front subfacies samples are between 0.25 and 0.61, with an average of 0.4. The C27/C27–29 S ratios of the shallow marine subfacies samples are between 0.40 and 0.60, with an average of 0.51 (Table 1). The C29/C27–29 S ratios of the delta front subfacies source rocks range from 0.28 to 0.56, with an average of 0.4 (Table 1). The C29/C27–29 S ratios of the shallow marine subfacies samples are 0.22–0.39, with an average of 0.30 (Table 1). C27, C28 and C29 rearranged steranes are widely developed in the studied samples. It is likely that the mud content was relatively high during the sedimentation period, and the clay minerals were relatively developed [45,46,47,48,49,50]. The parameters of the biomarker compound ratios are shown in Table 1.

5. Discussion

5.1. Distribution of Effective Source Rocks

Effective source rocks are defined as rocks that have generated and discharged large amounts of hydrocarbon fluids [3,22,31]. Therefore, it is important to determine effective source rocks. Because of the variations in the amount, type, and characteristics of OM found in various study regions, the criteria for effective source rocks cannot be unified. Different study areas have different standards for effective source rocks. During the production and ejection of hydrocarbons, the generated hydrocarbon should satisfy the source rock’s own adsorption and then discharge the excess hydrocarbons. Therefore, before reaching source rock saturation, TOC and S1 should have a good positive correlation. After reaching source rock saturation, with increasing TOC, S1 tends to stabilize. Before reaching source rock saturation, TOC and S1/TOC should have a good positive correlation, and S1/TOC gradually increases with increasing TOC. After reaching source rock saturation, S1/TOC gradually decreases with the beginning of hydrocarbon expulsion [21,31]. Based on this process, TOC versus S1/TOC and TOC versus S1 scatter plots were drawn to determine the effective source rocks of the E1l. TOC = 1.0 wt% is designated as the bottom limit of the effective source rocks. Referring to the evaluation criteria for marine source rocks proposed by scholars [21,31], TOC = 2 was defined as the lower limit of excellent source rocks. The detailed evaluation criteria are shown in Figure 7.
According to the source rock evaluation standards, the source rocks were evaluated. The evaluation results show that the source rocks of wells A, C, E, and F are well developed. The source rocks of wells B, D, and G are poorly developed. Wells A, C, E, and F are located in the delta front subfacies, and wells B, D, and G are located in the shallow marine subfacies. The statistics show that 75% of the source rocks from the delta front facies are good source rocks, while 12% are excellent source rocks, and 81% of the source rocks from the shallow marine facies are poor source rocks, while 19% are good source rocks (Figure 8). The delta front subfacies source rocks are obviously better than those from the shallow marine subfacies.

5.2. Sedimentary Environments

The Pr/Ph reflects the redox conditions of the depositional environment [51,52,53,54,55,56]. It is generally believed that pristane and phytane are derived from phytol in chlorophyll. Phytol is converted into phytane by dehydration and hydrogenation in reducing depositional environments. Phytane is firstly oxidized to phytanic acid, and phytanic acid is converted into pristane by dehydration and hydrogenation in oxidizing depositional environments. Pr/Ph ratios of <0.6, 0.6–3.0, and >3.0 correspond to reducing, sub-reducing to sub-oxidizing, and oxidizing depositional environments, respectively [51,52,53,54,55,57,58,59,60]. Gammacerane is believed to be the product of the reduction of tetrahydrofuran [61]. Bacterial ciliates are thought to be precursors to tetrahydrofuran [61,62]. Bacterial ciliates generally live in the transition zone between anoxic and aerobic zones [63,64]. Therefore, the gamma wax index (GI) is used to evaluate water stratification [65]. The C35/C34H ratio is used to indicate water salinity. Generally, the higher the salinity of the water body, the more the water body tends to restore the environment [56,66]. A graph of the relationship of each indicator was drawn. The figure shows that there is a negative correlation between GI and Pr/Ph with the R2 of 0.29. In addition, there is a poor positive correlation between C35/C34 H and the GI, with an R2 of 0.06. This indicates that the GI decreases with increasing Pr/Ph ratio (Figure 9A), and the C35/C34 H ratio increases with increasing GI (Figure 9B), which proves that these parameters are effective for evaluating the depositional environment. Each parameter indicates that the depositional environment of the delta front subfacies is generally a sub-oxidizing and oxic environment. However, due to the inconsistency between the freshwater charge of each delta and the water depth of the estuary, the water stratification and water salinity are differentiated. The shallow marine subfacies depositional environment is sub-reducing and suboxic, with a high salinity and relatively stable water stratification.

5.3. Organic Matter Sources

The short-chain n-alkanes (nC14–nC20) are derived from bacteria and algae, and the low-chain n-alkanes (nC26–nC35) are derived from terrestrial higher plants [67,68,69,70]. Terrigenous input level is calculated using the ratios nC15+17+19/nC27+29+31, nC27+29/nC31+33, average chain length (ACL), and aquatic microphyte (Paq), with high values of ACL indicating weak terrigenous OM input and low values of ACL indicating strong terrigenous OM input [71]. The results show that the delta front subfacies source rocks have low values of nC15+17+19/nC27+29+31, nC27+29/nC31+33 and Paq, with high values of ACL [72,73]. In contrast, the shallow marine subfacies source rocks have high values of nC15+17+19/nC27+29+31, nC27+29/nC31+33 and Paq and low value of ACL (Figure 10). This indicates that the delta front subfacies source rocks have source inputs mainly from terrestrial high plants, and the source of OMs of shallow marine subfacies source rocks are mainly from algae [68,69,70].
The sources of tricyclic terpenes and tetracyclic terpenes are complex, and it is now agreed that C19 TT is related to the input of terrestrial higher plants, and C23 TT is mainly derived from aquatic organisms [74]. The C24 TeT is abundant in terrigenous samples [4,8,14,21,31,74]. Figure 11A shows that the delta front subfacies source rocks have high values of the two parameters, while the shallow marine subfacies source rocks have lower values. Oleanane is mainly derived from post-Cretaceous angiosperms [66,75]. The higher oleanane index values for the delta source rocks than the shallow marine subfacies source rocks suggest that the former have more angiosperm inputs than the latter (Table 1). The lower aquatic creatures and algae are the source of the C27 regular steranes. The regular C29 steranes come from higher terrestrial plants [45,48,50,76,77]. The data show that the shallow marine subfacies source rocks have a higher relative content of C29 regular steranes than the delta front subfacies source rocks (Figure 11B–D).
Based on the composition of n-alkanes, terpenoids and regular steranes, the OM of the delta front subfacies source rocks have source inputs mainly from terrestrial high plants; in contrast, the shallow marine subfacies source rocks have source inputs mainly from algae. The source differentiation of OM may be controlled by sedimentary facies. The delta front subfacies not only incorporates terrestrial OM input from rivers, but also in situ marine OM accumulation. The shallow marine subfacies is far from the provenance area and has a lighter input of only leaf wax, and the supply of terrigenous higher plants is poor. The algae that developed in situ are the source of OM. This development is due to the high salinity of neritic water and stable water stratification.

5.4. Organic Matter Accumulation

5.4.1. Influence of Organic Matter Input

OM input is the basic condition for OM enrichment. Marine source rocks have various sources of OM [6,7,14]. OM includes terrestrial OM debris transported by rivers, lower aquatic organisms developed in transitional facies, and planktonic algae and bacteria developed in the sea [75]. The contribution of different sources of OM to OM enrichment not only determines the pattern of OM enrichment but also controls the development location of excellent source rocks [6,7,8]. N-alkanes are the most abundant components in biomarker compounds [46,78,79,80].
Scatter plots were plotted for each biomarker parameter and TOC. The figure shows that TOC is inversely proportional to the nC15+17+19/nC27+29+31, nC27+29/nC31+33 and Paq, the R2 values of the nC15+17+19/nC27+29+31, nC27+29/nC31+33 and Paq are 0.14, 0.24 and 0.18 (Figure 12A–C). With increasing nC15+17+19/nC27+29+31, nC27+29/nC31+33 and Paq, the TOC tends to increase. In addition, TOC is proportional to the ACL, with an R2 of 0.22 (Figure 12D). As ACL increases, TOC significantly decreases.
The scatter plot of the C24 TeT/C23 TT ratio and TOC shows that the delta front subfacies source rocks and the shallow marine subfacies source rocks have the same characteristics. The C24 TeT/C23 TT are significantly positively correlated with TOC with the R2 of 0.23 (Figure 13A). With increasing C24 TeT/C23 TT, the TOC tends to increase. The oleanane index and TOC have a relationship that is consistent with C24 TeT/C23 TT. There is a poor positive correlation between TOC and the oleanane index, with an R2 of 0.11. TOC increases dramatically when the oleanane index rises (Figure 13B). The relationship between C27/C27–29 S and TOC is obviously opposite to that of C29/C27–29 S and TOC (Figure 13C,D). With increasing C27/C27–29 S, the TOC tends to decrease. With increasing C29/C27–29 S, TOC significantly increases.
In summary, the OM enrichment of source rocks is affected by the input of terrigenous OM. The supply index of TOM (nC15+17+19/nC27+29+31, nC27+29/nC31+33, Paq, C24 TeT/C23 TT and C29/C27–29 S) has a good relationship with TOC, the R2 values of the nC15+17+19/nC27+29+31, nC27+29/nC31+33, Paq, C24 TeT/C23 TT and C29/C27–29 S are 0.14, 0.24, 0.18, 0.23 and 0.31. The greater the TOM input, the greater the TOC of the source rocks. This phenomenon implies that the key element influencing the origin of the source rocks is the supply of TOM.

5.4.2. Influence of Organic Matter Preservation

OM preservation has a significant impact on the enrichment of OM. OM preservation is controlled by the redox degree of the sedimentary environment and the salinity of water [14]. OM is deactivated in oxidizing environments, which is not conducive to the preservation of OM and the origin of source rocks [14,80,81]. Scatter plots of each parameter of OM preservation and TOC were plotted. The scatter plot of Pr/Ph and TOC shows no obvious relationship between TOC and Pr/Ph (Figure 14). This phenomenon may be because when the supply of TOM is large, the environment is oxic. Furthermore, the content of original OM is high. Thus, the OM was deposited at depth and converted into excellent source rocks, even if several of the OM degraded.

5.4.3. Models for Effective Source Rock Formation

During the deposition of the Paleocene E1l in the Lishui Sag, the climate was warm and humid [22,31]. In addition, we identified the supply of detrital input from previous works. Li et al. pointed out that the average relative content of (Fe + K) in the E1l was 7.74% and the sediment flux in the E1l was 158 t/km2/yr [82]. This indicates that the number of terrestrial inputs is large. Source rocks mainly developed in the delta front subfacies and shallow marine subfacies. Although the redox conditions of the delta front subfacies fluctuated greatly, the delta front consisted of a sub-oxidizing environment, providing good degradation of OM. The supply of terrigenous OM to the delta front subfacies was abundant. The amount of OM in the water is large. At the same time, the time of the exposure of OM to oxygen was reduced by the high sediment flux [1,75,82]. The OM was deposited at depth and converted into excellent source rocks. Previous works have also pointed out that TOM provided from rivers is an important factor that is critical to organic accumulation [1,35,75]. The shallow marine subfacies is far from the source supply area. Although the shallow sea subfacies redox environment was dominated by weak reduction, it was conducive to the preservation of OM. The input of TOM was small, and excellent source rocks were not developed. The input of TOM controlled the enrichment of OM in the E1l source rocks. The OM enrichment model of the E1l source rocks can be defined as the delta front-OM input type (Figure 15).

6. Conclusions

In this study, the source rocks of the Paleocene E1l in the Lishui Sag were evaluated on the basis of TOC, rock pyrolysis and biomarker compounds. The OM source and depositional environment of the source rocks were studied. The factors controlling the variable abundance of OM in the Lingfeng Formation were studied using organic geochemical data. A model of OM enrichment was developed following examining the key variables influencing the OM input and preservation conditions. The conclusions drawn were as follows:
(1) The source rocks of the E1l had medium–high TOC (range: 0.53–3.56%, average: 1.16%). TOC = 1.0 wt% was determined as the bottom limit of the effective source rocks. TOC = 2 was determined as the lower limit of excellent source rocks. The evaluation results show that the number of excellent source rocks in the delta front subfacies was obviously greater than that in the shallow marine subfacies.
(2) Based on the data of n-alkanes, terpenoids and regular steranes, the depositional environment and OM source were analyzed. The OM in the source rocks of the delta front subfacies mainly came from terrestrial and aquatic plants, and the depositional environment consisted of oxidative–weakly reducing conditions. The OM of the shallow marine subfacies source rocks mainly came from planktonic algae, and the depositional environment consisted of weakly reducing conditions.
(3) The OM enrichment of the E1l in the Lishui Sag was controlled by the input of terrigenous OM. Terrigenous higher plants, as nutrients, promoted the accumulation of OM and the development of excellent source rocks. Preservation conditions were not the main factor controlling OM enrichment. A delta front–OM input type for the formation of these excellent source rocks is proposed.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/en16042046/s1, Table S1: Geochemical parameters of the samples from the Lingfeng Formation.

Author Contributions

Conceptualization, X.H.; data curation, X.C. and Y.L.; formal analysis, X.H.; investigation, X.C.; methodology, X.H. and D.H.; resources, D.H.; software, Y.L.; supervision, D.H.; writing—review & editing, X.H. All authors have read and agreed to the published version of the manuscript.

Funding

The National Natural Science Foundation of China under contract No. 41872131.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (A) Tectonic location of the East China Sea Shelf Basin (ECSSB). (B) Structural units of the Lishui Sag. A: Well A; B: Well B; C: Well C; D: Well D; E: Well E; F: Well F; G: Well G.
Figure 1. (A) Tectonic location of the East China Sea Shelf Basin (ECSSB). (B) Structural units of the Lishui Sag. A: Well A; B: Well B; C: Well C; D: Well D; E: Well E; F: Well F; G: Well G.
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Figure 2. (A) Sedimentary subfacies plan of the Lingfeng formation in the Lishui Sag; (B) stratigraphic units of the Paleocene in the Lishui Sag. A: Well A; B: Well B; C: Well C; D: Well D; E: Well E; F: Well F; G: Well G.
Figure 2. (A) Sedimentary subfacies plan of the Lingfeng formation in the Lishui Sag; (B) stratigraphic units of the Paleocene in the Lishui Sag. A: Well A; B: Well B; C: Well C; D: Well D; E: Well E; F: Well F; G: Well G.
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Figure 3. Plot of HI versus Tmax based on rock pyrolysis. A: kerogen type of Well A; B: kerogen type of Well B; C: kerogen type of Well C; D: kerogen type of Well D; E: kerogen type of Well E; F: kerogen type of Well F; G: kerogen type of Well G.
Figure 3. Plot of HI versus Tmax based on rock pyrolysis. A: kerogen type of Well A; B: kerogen type of Well B; C: kerogen type of Well C; D: kerogen type of Well D; E: kerogen type of Well E; F: kerogen type of Well F; G: kerogen type of Well G.
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Figure 4. (A) Mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well A; (B) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well B; (C) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well C; (D) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well D; (E) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well E; (F) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well F; (G) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well G.
Figure 4. (A) Mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well A; (B) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well B; (C) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well C; (D) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well D; (E) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well E; (F) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well F; (G) mass chromatograms of the n-alkanes (m/z 85) of the saturated hydrocarbons in Well G.
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Energies 16 02046 g005 550
Figure 5. (A) Mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well A; (B) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well B; (C) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well C; (D) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well D; (E) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well E; (F) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well F; (G) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well G. Ts: C27 22,29,30-trisnorneohopane; Tm: C27 22,29,30-trisnorhopane.
Figure 5. (A) Mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well A; (B) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well B; (C) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well C; (D) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well D; (E) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well E; (F) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well F; (G) mass chromatograms of the n-alkanes (m/z 191) of the saturated hydrocarbons in Well G. Ts: C27 22,29,30-trisnorneohopane; Tm: C27 22,29,30-trisnorhopane.
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Figure 6. (A) Mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well A; (B) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well B; (C) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well C; (D) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well D; (E) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well E; (F) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well F; (G) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well G.
Figure 6. (A) Mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well A; (B) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well B; (C) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well C; (D) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well D; (E) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well E; (F) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well F; (G) mass chromatograms of the n-alkanes (m/z 217) of the saturated hydrocarbons in Well G.
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Figure 7. Cross-plot of TOC versus. S1 + S2.
Figure 7. Cross-plot of TOC versus. S1 + S2.
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Figure 8. Source rock evaluation results.
Figure 8. Source rock evaluation results.
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Figure 9. (A) The plot of the gammacerane index versus Pr/Ph; (B) The plot of the gammacerane index versus C35/C34 H.
Figure 9. (A) The plot of the gammacerane index versus Pr/Ph; (B) The plot of the gammacerane index versus C35/C34 H.
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Figure 10. (A) The plot of nC15+17+19/nC27+29+31 versus nC27+29/nC31+33; (B) plot of Paq versus ACL.
Figure 10. (A) The plot of nC15+17+19/nC27+29+31 versus nC27+29/nC31+33; (B) plot of Paq versus ACL.
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Figure 11. (A) The plot of C24 TeT/C23 TT versus C19 TT/C23 TT; (B) plot of C27/C27–29 S versus C29/C27–29 S; (C) plot of oleanane/C30 hopane versus C29/C27–29 S; (D) plot of C24 TeT/C23 TT versus C29/C27–29 S.
Figure 11. (A) The plot of C24 TeT/C23 TT versus C19 TT/C23 TT; (B) plot of C27/C27–29 S versus C29/C27–29 S; (C) plot of oleanane/C30 hopane versus C29/C27–29 S; (D) plot of C24 TeT/C23 TT versus C29/C27–29 S.
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Figure 12. Relationship between OM supply index and TOC content of source rocks. (A) The relationship between TOC and the Paq; (B) relationship between TOC and the nC15+17+19/nC27+29+31; (C) relationship between TOC and the nC27+29/nC31+33; (D) relationship between TOC and the ACL.
Figure 12. Relationship between OM supply index and TOC content of source rocks. (A) The relationship between TOC and the Paq; (B) relationship between TOC and the nC15+17+19/nC27+29+31; (C) relationship between TOC and the nC27+29/nC31+33; (D) relationship between TOC and the ACL.
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Figure 13. Relationship between OM supply index and TOC content of source rocks. (A) The relationship between TOC and C24 TeT/C23 TT; (B) relationship between TOC and oleanane/C30 hopane; (C) relationship between TOC and the regular steranes C27/C27–29; (D) relationship between TOC and the regular steranes C29/C27–29.
Figure 13. Relationship between OM supply index and TOC content of source rocks. (A) The relationship between TOC and C24 TeT/C23 TT; (B) relationship between TOC and oleanane/C30 hopane; (C) relationship between TOC and the regular steranes C27/C27–29; (D) relationship between TOC and the regular steranes C29/C27–29.
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Figure 14. The relationship between the Pr/Ph and TOC.
Figure 14. The relationship between the Pr/Ph and TOC.
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Figure 15. Organic matter enrichment model of the Lingfeng Formation source rocks.
Figure 15. Organic matter enrichment model of the Lingfeng Formation source rocks.
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Table
Table 1. Selected biomarker parameters for the Lingfeng Formation samples.
Table 1. Selected biomarker parameters for the Lingfeng Formation samples.
WellDepth (m)TOC (%)C19/C23TTC20/C23TTC24TeT/C23TTC27/C27–29 SC29/C27–29 SPr/PhC35H/C34HTm/C30HGam/C30 HTs/TmOle/C30 HPaqACLnC27+29/nC31+33nC15+17+19/nC27+29+31
A23701.070.40.60.930.310.411.91/0.460.150.290.110.4127.741.470.9
28801.320.640.841.270.330.474.77/0.690.060.060.070.9524.581.285.91
30001.280.20.431.220.350.453.990.480.60.080.070.170.3328.041.250.18
30101.510.390.721.240.310.494.520.510.70.080.060.110.2728.351.210.13
31352.420.330.3910.250.563.930.210.680.090.070.110.3128.131.390.14
31921.760.170.320.260.270.532.780.340.540.080.080.10.3328.021.420.13
28501.50.120.630.970.380.371.270.520.20.210.740.080.4627.321.11
29451.250.080.390.830.370.381.210.630.230.170.730.090.5427.062.152.01
30401.060.150.240.870.350.391.320.520.220.220.540.080.5427.042.251.9
B3235.50.850.560.710.950.580.32.920.490.170.040.610.050.6726.422.631.88
33331.041.121.211.510.450.353.180.410.140.030.710.080.726.2331.96
3347.81.030.831.061.180.460.342.420.460.090.041.150.070.7426.232.642.57
3520.51.020.410.980.640.590.292.280.610.330.110.240.010.6326.752.260.71
3616.51.210.590.970.720.420.392.520.50.120.081.050.030.6726.322.981.07
37571.140.580.850.640.410.342.750.50.140.110.890.040.6926.32.921.47
C2916.51.190.250.370.20.360.412.570.520.230.080.450.10.5627.431.170.74
33372.310.310.590.190.480.363.26/0.310.120.420.130.6326.742.181
35801.420.560.90.170.440.362.01/0.430.160.530.110.7226.372.441.65
36051.510.820.940.170.410.391.07/0.740.150.220.090.727.011.151.54
33651.640.393.440.580.410.380.630.480.270.150.550.130.5127.052.290.55
35751.430.350.50.350.360.391.810.330.310.20.450.080.5826.951.980.83
D22980.990.860.860.710.540.263.420.530.160.030.580.030.5626.63.280.59
3152.50.880.440.810.690.570.222.970.50.150.060.750.050.6726.213.521.04
3257.50.850.470.590.910.40.332.470.610.130.051.550.150.726.232.811.36
E30550.860.110.330.290.420.331.270.150.280.210.590.080.6826.422.090.91
31360.670.190.330.160.510.291.12/0.390.160.830.080.8125.493.560.63
3337.61.510.620.680.230.570.33.39/0.170.111.250.250.5527.051.890.64
3339.61.290.810.950.20.580.313.78/0.150.121.50.410.57271.930.84
3349.40.861.090.820.180.620.283.69/0.160.131.380.310.626.841.90.78
33880.840.220.290.290.420.351.760.620.230.210.910.070.5826.742.30.87
F26100.810.430.631.350.280.53.35/0.490.120.090.060.3927.421.750.2
26451.190.841.061.680.320.474.610.360.420.150.170.090.427.521.720.27
26651.330.490.661.910.360.434.8100.610.080.10.050.3827.391.990.22
26751.060.680.661.20.360.414.630.390.390.130.120.080.4127.661.540.39
26351.10.80.650.270.330.452.180.330.490.110.10.070.4527.471.60.42
26701.171.050.81.740.330.442.450.310.590.10.10.060.4827.411.640.34
27051.240.980.751.990.330.452.340.330.530.110.120.070.4927.351.470.48
27341.230.830.531.510.320.452.410.340.430.120.160.080.4127.252.120.33
G30750.790.710.861.110.490.323.550.430.240.030.390.060.6726.462.391.79
3181.50.650.921.191.210.60.313.410.70.240.030.420.070.7226.042.861.56
35880.550.530.750.670.540.282.620.50.090.042.710.140.7425.982.621.97
3372.50.761.21.51.210.570.223.570.310.510.050.290.130.7725.723.222.49
34890.871.021.261.020.580.243.270.30.660.040.130.070.7126.152.951.68
TT: tricyclic terpanes; TeT: tetracyclic terpanes; C27/C27–29 S: C27/C27–29 sterane; C29/C27–29 S: C29/C27–29 sterane; Pr/Ph: pristane/phytane; C35/C34 H= C35 22 S/C34 22 S hopane; Ts: C27 22,29,30-trisnorneohopane; Tm: C27 22,29,30-trisnorhopane; Gam/C30 H: gammacerane/αβ-C30 hopane;Ole: oleanane;Paq: (nC23 + nC25)/(nC23 + nC25 + nC29 + nC31);ACL:(23 × nC23 + 25 × nC25 + 27 × nC27 + 29 × nC29 + 31 × nC31 + 33 × nC33)/(nC23 + nC25 + nC27+nC29 + nC31 + nC33).
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Han, X.; Hou, D.; Cheng, X.; Li, Y. Impact of Terrigenous Organic Matter Input on Organic Matter Enrichment of Paleocene Source Rocks, Lishui Sag, East China Sea. Energies 2023, 16, 2046. https://doi.org/10.3390/en16042046

AMA Style

Han X, Hou D, Cheng X, Li Y. Impact of Terrigenous Organic Matter Input on Organic Matter Enrichment of Paleocene Source Rocks, Lishui Sag, East China Sea. Energies. 2023; 16(4):2046. https://doi.org/10.3390/en16042046

Chicago/Turabian Style

Han, Xu, Dujie Hou, Xiong Cheng, and Yan Li. 2023. "Impact of Terrigenous Organic Matter Input on Organic Matter Enrichment of Paleocene Source Rocks, Lishui Sag, East China Sea" Energies 16, no. 4: 2046. https://doi.org/10.3390/en16042046

APA Style

Han, X., Hou, D., Cheng, X., & Li, Y. (2023). Impact of Terrigenous Organic Matter Input on Organic Matter Enrichment of Paleocene Source Rocks, Lishui Sag, East China Sea. Energies, 16(4), 2046. https://doi.org/10.3390/en16042046

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