Geochemistry and geological significance of the Upper Paleozoic and Mesozoic source rocks in the Lower Yangtze region

Chin.J.Geochem.(2012)31:315–322 DOI: 10.1007/s11631-012-0581-3

www.gyig.ac.cn www.springerlink.com

Geochemistry and geological significance of the Upper Paleozoic and Mesozoic source rocks in the Lower Yangtze region HUANG Yanran1,2*, ZHANG Zhihuan1, WU Liyuan3, ZANG Chunjuan1, and LI Qiong1

1 State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China 2 Hunan Province Key Laboratory of Coal Resources Clean-utilization and Mine Environment Protection, Hunan University of Science and

Technology, Xiangtan 411201, China 3 Sinopec East China Company, Nanjing 210011, China

* Corresponding author, E-mail: [email protected]

Received April 13, 2011; accepted May 15, 2011 © Science Press and Institute of Geochemistry, CAS and Springer-Verlag Berlin Heidelberg 2012

Abstract The Lower Yangtze region is one of the important marine sedimentation areas of oil and gas distribution in southern China, for its favorable source rocks, reservoirs and covers. However, the intense tectonic movements and complex hydrocarbon generation process made it highly impossible to form large-sized oil and gas reservoirs. So it was divided to different hydrocarbon-bearing preservation units in oil-gas exploration. Recent study shows that the Permian and Lower Triassic source rocks in the Lower Yangtze region are complicated in lithology. The hydrocarbon generation potential of limestone there is low while argillaceous source rocks are overall of high abundance with excellent organic types, now in the process of hydrocarbon generation, so differences in high maturity influence the evaluation of organic matter abundance and type. Biomarker characteristics indicate a reductive environment. n-alkanes are marked by a single peak, with no odd-even predominance. The composition and distribution of the carbon numbers of n-alkanes, and the high abundance of long-chain tricyclic terpanes are indicative of marine sedi-mentation. The high contents of pregnane, homopregnane, rearranged hopane suggest that the source rocks are of high maturity. There is a good linear correlation between methylphenanthrene index and vitrinite reflectance. The correlation of oil-source rocks indicated that the oil of Well HT-3 may come from the Permian Longtan Formation in the Huangqiao area, the oil of Wells Rong-2 and Juping-1 came from the Lower Triassic Qinglong Formation in the Jurong area. The exploration here is promising in those different source rocks which all have great potential in hy-drocarbon generating, and oil and gas were produced in the late stage of hydrocarbon generation. Key words high maturity; source rock evaluation; oil-gas correlation; Lower Yangtze region

1 Introduction

The Lower Yangtze region is located in the east of the Yangtze plate, covering an area of nearly 230000 km2. It extends to the Tancheng-Lujiang fault in the North China plate in the northwest, and is bounded in the Huaxia apophysis by the Jiang-shan-Shaoxin fault in the southeast. In the region are developed several sets of source-reservoir-cap combi-nation, so the original conditions are favorable to hy-drocarbon generation, migration, and accumulation (Wang Genhai, 2000). Meanwhile, the region has ex-perienced Caledonian, Hercynian, Indosinian, Yan-shanian, Himalayan and other multi-stage transforma-tions by tectonic movements, which ended with no basin function there, hence it can be called the multi-stage structural superimposed residual basin (Liu

Guangding, 2001). After a long time of research and exploration practice, people have recognized that the storage conditions are critical to a petroleum system. So the new method of evaluation, also the study premise for this area, is developed, enduing with “an oil and gas preservation unit” (Liang Xing et al., 2004), which helps divide the reservoir units in terms of the key elements of oil and gas sources, Previous studies suggested that the Permian and Lower Triassic source rocks are the main source rocks in the Lower Yangtze area and the marine strata have a high abun-dance of organic matter and favorable conditions for oil generating (Xiao Kaihua et al., 2006; Chen Anding et al., 2001; Liang Digang et al., 2008). However, al-most no profitable industrial purpose was made. The main source rocks in different hydrocarbon-generating areas and oil and gas sources are still expected to be

316 Chin.J.Geochem.(2012)31:315–322

found. This paper intended to systematically study the geochemical characteristics of main source rocks and make oil-source correlation, so as to look for the geo-logical significance, providing evidence for oil and gas exploration.

2 Geological background

The Lower Yangtze region underwent three main tectonic evolution movements, including the primitive strata formation and constructional stable phase (Z–T1), the squeezing rebuilt and superimposed pres-sure phase (T2–K1), and the transformation of alter-nating tension and extrusion, composite overlay bend off stage (K2-Present) (Yang Shenliang, 1997). The latest tectonic movement forming two major tectonic units continued to deposit as the main feature in South Jiangsu, and upleft to corrode in North Jiangsu, with increasing denudation extent from north to south.

Tectonic movements varied in different stages, causing differences in the burial history. The source rocks did not experience a simple burial, but several phases of uplift and erosion in different degrees. Therefore, the hydrocarbon generation is complex, generally with the characteristics of secondary or late hydrocarbon generation. Oil and gas resource evalua-tion showed that the Huangqiao (HQ) and Jurong (JR) areas are potentially hydrocarbon-rich areas, where a large number of low-yielding oil, gas, and oil flows were found. The HQ area is located in the hedge zone of Tongling - Nanjing - Zhenjiang - Taizhou - Dongtai, showing a normal stratigraphic sequence. And the JR region lies in the south-to-north thrust fault zone, with the reversed and repeated stratigraphic succession. Oil and gas in both areas mainly came from the Permian and Lower Triassic source rocks (Guo Nianfa and Lei Yixin, 1998), for their geochemical characteristics are similar. The hydrocarbon generation and expulsion processes of source rocks are complex, and the main source layer is not very distinct. In this study there were collected crude oil samples from Wells HT-3 and Rong-2 and a lot of source rock samples from the new low-yielding well Juping-1, allowed by the Sinopec East China Company for reviewing and testing their geochemical analyses.

3 Geochemical characteristics of source rocks

3.1 The distribution, potential and revolution of source rocks

The prime source rock layers of Upper Paleozoic and Mesozoic in the Lower Yangtze region, from bottom to top, are respectively the Qixia, Gufeng,

Longtan, Dalong formations in the Permian, and the Qinglong Formation in the Lower Triassic. The thick-ness of the source rock layer was restored mainly by stratigraphic correlation. Lithologically, the Qixia Formation is primarily composed of a large set of limestone with the maximum total thickness up to 500 m, including some bioclastic and stink limestone, oc-casionally thin coal seams. And the Longtan Forma-tion is composed mainly of coal-bearing mudstone and several sets of compact sandstones, with a large stratigraphic thickness being generally about 150 m. The Gufeng Formation consists mainly of black mud-stone, carbonaceous mudstone and coal-seams, and the Dalong Formation consists of siliceous mudstone, both of which are relatively small in thickness. The upper Qionglong Formation is composed of dark gray limestone at the top, and black thin mudstone and limestone at the bottom, with the thickness up to 1000 m or more. The sedimentary environment of the source rocks displays an evolutionary process from platform marine to transitional environment, epicon-tinental marine, transitional environment and platform marine environment. The source rocks in each layer varied greatly in lithology, including shale, limestone, marl, carbonaceous mudstone, coal, etc., which re-quires different evaluation criteria according to their different lithological features.

Fig. 1. The correlation diagram between residual organic carbon and

hydrocarbon potential in different formations of the Permian and

Lower Triassic source rocks.

Source rock evaluation parameters are listed in

Tables 1 and 2. The target zone of the JR area is the Triassic Qinglong Formation, only a few samples were collected from the Permian system, and the car-bonaceous mudstone and coal samples from the Gufeng Formation could not be able to be collected, thus only mudstone sample parameters for the Gufeng Formation are listed in the tables. Influenced by high thermal evolution, the chloroform bitumen "A" pro-duction index (PI) and hydrogen index (HI) are low. While compared to the overall organic matter, the or-ganic carbon contents are relatively low, and have a

Chin.J.Geochem.(2012)31:315–322 317

good correspondence with its hydrocarbon potential (Fig. 1), which can be used as an effective means to evaluate the high maturity source rocks. On the whole, the other layers are good source rocks except the limestone of the Qixia and Qinglong formations. It is worthy of notice that the hydrocarbon potential of the Qinglong Formation limestone is low with the organic carbon contents accounting for 0.12% and 0.22% be-tween HQ and JR area, which is the main hydrocarbon generating section of the argillaceous layer. Maximum pyrolysis temperature (Tmax) reflects the thermal evo-lution of source rocks, the more upper the layer of hydrocarbon source rocks, the smaller the Tmax value will be. The correlation between HI and Tmax shows that the main organic matter type is type III, which is mainly due to faster cracking of sapropel organic matter than that of the humic type (Chen Keming and Wang Zhaoyun, 1996; Dembicki, 2009), and tends to demonstrate humification in the late period of oil gen-eration, when the sapropel organic matter was in a small proportion of kerogen.

3.2 Organic petrology characteristics

The measured parameters of vitrinite reflectance (R0) and methylphenanthrene index for the Permian and Lower Triassic source rocks have a good linear correlation (Fig. 2). Other R0 values of the source

rocks can be calculated according to the methylphe-nanthrene parameter. Whether measured or calculated, R0 values vary mainly within the range of 1%–1.4% (Fig. 3), indicating high maturity. Microscopic char-acteristics showed that the main type of organic matter is type Ⅲ, and subordinately Ⅱ2. The principal source of hydrocarbons is algal debris mass, giving rise to low contents of organic matter content, usually 0.8%–3%. And many microscopic forms of organic matter in limestone samples can not be observed, be-cause of poor hydrocarbon potential. Vitrinite macer-als in the main samples mostly account for more than 80%, and in some samples they are even up to 100%, the contents of inertinite and liptinite are very low, probably because of high maturity, and light features disappeared and are difficult to find. This phenome-non was also observed in the same layer of the other Yangtze region (Liang Digang et al., 2007). Vitrinite component is mainly massive, clastic telocollinite, which has a wide anisotropy (△R=0.94%–1.61%). The leached oil was gray under reflected light, with little or basically no fluorescence. And prevalence of the secondary components was mainly present in the forms of secondary asphalt, organic inclusions and oil droplets; sometimes the thermal evolution-induced secondary components was appeared in somewhere (Zhang Youshen, 2003), reflecting secondary hydro-carbon generation.

Table 1 List of organic geochemical parameters for the Permian and Lower Triassic source rocks in the HQ area

Formation and lithology

Interval value

TOC (%)

S1+S2 (mg/g)

Chl “A” (%)

PI (%)

HI (mg/g)

Tmax (℃)

Max 0.47 1.25 0.11 0.27 67 513

Min 0.01 0.01 0.01 0.04 7 435 T1qn

(limestone) Average 0.12 0.35 0.03 0.14 21.5 465

Max 3.38 17.16 0.57 0.73 80 493

Min 0.11 0.11 0.01 0.18 26 423 T1qn

(mudstone) Average 1.4 4.69 0.15 0.35 33.4 451

Max 5.42 4.59 0.05 0.34 241 483

Min 0.22 0.01 0.02 0.08 5 435 P2d


Max 4.14 25.43 0.1 0.5 266 520

Min 0.38 0.25 0.02 0.05 11 438 P2l


Max 5.87 2.47 0.24 0.24 59 520

Min 0.96 0.32 0.01 0.07 13 444 P1g


Max 58.16 28.28 0.36 0.3 123 546

Min 6.81 0.93 0.06 0.08 21 456 P1g

(carbonaceous mudstone)

Average 14.32 8.65 0.25 0.18 65.4 479

Max 1.27 0.58 0.24 0.19 40 539

Min 0.15 0.07 0.01 0.01 14 466 P1q


318 Chin.J.Geochem.(2012)31:315–322

Table 2 List of organic geochemical parameters for the Permian and Lower Triassic source rocks in the JR area Formation

and lithology Interval value

TOC (%)

S1+S2 (mg/g)

Chl “A” (%)

PI (%)

HI (mg/g)

Tmax (℃)

Max 0.37 0.28 0.11 0.32 200 511

Min 0.05 0.11 0.01 0.11 28 434 T1qn


Max 5 3.53 0.28 0.6 275 455

Min 0.31 0.16 0.07 0.17 16 428 T1qn


Max 2.8 3.15 0.5 0.34 83 447

Min 1.6 1.03 0.02 0.15 18 433 P2d

(mudstone) Average 2.1 2.09 0.16 0.25 51 439

Max 3.9 6.45 0.6 0.6 113 452

Min 0.5 0.09 0.03 0.03 12 435 P2l


Max 4.3 0.82 0.11 0.31 18 516

Min 1.3 0.45 0.01 0.15 11 439 P1g


Max 0.93 0.79 0.2 0.38 42 518

Min 0.23 0.12 0.01 0.25 10 435 P1q


Special attention should be paid to the fact that mineral-based asphalt is more often observed than what is present in the form of organic matter in many samples, such as in the Longtan Formation, the mud-stone sample from Well Xi-2 in the HQ area contains 88.2% mineral-based asphalt, and in the Qinglong Formation, the mudstone sample from Well Ju-3 in the JR area contains 67.5% mineral-based asphalt, indi-cating that the source rocks are in the stage of hydro-carbon generation. Vitrinite in rocks is often present in the hidden form of finely dispersed organic matter. Different from other types of organic matter or miner-als, it was seen as the secondary occurrence and gave off strong yellow or yellow-green fluorescence, which is mainly due to hydrocarbon generation and expul-

sion. So, the phenomenon caused by hydrocarbon generation and migration does not represent the hy-drogen-rich hydrocarbon components.

3.3 Group composition and carbon isotopes

The group composition of chloroform bitumen "A" in the source rocks is affected by the sources of organic matter, storage conditions, maturity and other factors. Sapropelic organic matter is enriched in satu-rated hydrocarbons and aromatics, and humic organic matter is enriched in non-hydrocarbon and asphaltene (Ourisson et al., 1987). The contents of each compo-nent are of important reference value, for example, the correlation between the ratio of saturated hydrocar-

Chin.J.Geochem.(2012)31:315–322 319

bons over aromatic hydrocarbons and the contents of saturated hydrocarbons and aromatics can be used to constrain the original sources and types of organic matter (Fig. 4). It can be seen from the figure that or-dinary source rocks have the characteristics of type-II kerogens, but a considerable number of the Qinglong Formation mudstone samples have the characteristics of type-I kerogens, which indicates that the original organic matter contains a considerable amount of sapropelic organic matter, and the proportions of saturated hydrocarbons and aromatic hydrocarbons take up fair percentages of the samples in the rela-tively low maturity source rocks of the Qinglong Formation. The group composition of the source rocks was influenced by thermal evolution.

The group composition of the extracts from source rocks is regular with respect to δ13C values. The δ13C values of chloroform bitumen "A" vary from -26.8‰ to -29.3‰, those of saturated hydro-carbons vary between -21.4‰ and -31.2‰, those of the aromatics range from -20.1‰ to -29.6‰, those of non-hydrocarbons vary between -21.9‰ and -29.4‰, and those of asphaltenes vary between -19.3‰ and -29.6‰. The δ13C values of chloroform bitumen "A" vary over a very small range, following the progres-sively increasing order (δ13C) of saturated hydrocar-bons → aromatics → non-hydrocarbon → asphalte-nes. The δ13C values of saturated hydrocarbons in some samples are greater than those of non-aromatic hydrocarbons and asphaltenes. This may be due to the different sources of their parent materials.

Fig. 4. The relation among the extracted components of the Permian

and Lower Triassic source rocks. I, II, and III are different types of

organic matter.

3.4 Biomarker characteristics

Biomarker is an effective parameter to constrain the source of organic matter, the environment of deposition, the maturity of organic matter, etc. (Mol-dowan et al., 1985). During the Permian and Triassic, the HQ and JR areas in the Lower Yangtze region were in a regression-transgression-regression alter-

nating sedimentary environment. The sources of or-ganic matter are complex, while the biomarker char-acteristics of source rocks in different layers are quite similar. For most of the samples, the carbon numbers of n-alkanes range from nC12 to nC35, mainly on a single peak with the former peak shape, with the main peak of carbon appearing at nC14 to nC18. The abun-dance of low-carbon alkanes is high and mainly af-fected by the combination of source and high maturity, with no odd-even predominance. Pr/Ph values in the samples vary mainly between 0.8 and 1.5, and some are even less than 0.5, showing a reducing environ-ment in this area. The conditions in the JR area are even better. Almost no β-carrot alkyl was found in saturated hydrocarbons. The carbon numbers of tri-cyclic terpanes series are widely distributed and their abundance is high, suggesting they were derived from marine algae and microbes. The contents of C23 tri-cyclic terpane are usually highest and those of long-chain tricyclic terpanes (C27-C30) are relatively high, which are the most important features differing from those of a continental deposit. The concentra-tions of gammacerane are low. The ratios of gammac-erane over C30 hopane range from 0.2 to 0.3 in the HQ area, and are slightly lower in the JR area, which re-flects the normal abundance of gammacerane in ma-rine sediments. Usually, the relative contents of C27-C28-C29 steranes can indicate the biological source of organic matter, with aquatic rich in C27 and C28 steranes, and C29 steranes were derived from higher plants (Jacob, 1989; Peters et al., 2005). The contents of C27 and C29 steranes are high in the Permian and Lower Permian source rocks, generally varying be-tween 30% and 45%, while C28 sterane is relatively low. C29 sterane has an advantage in the Early Paleo-zoic when there exists no higher plants source (Zeng Fangang et al., 1998). Therefore, based on these re-sults, the sources of organic matter can not be deter-mined, and C29 sterane may come from the uncon-firmed sources of marine cyanobacteria or alginic.

Dicyclic sesquiterpenoid alkanes mainly consist of C14-C16 sesquiterpanes series which is based on drimane skeleton, including two of C14, four of C15, two of C16, 8α(H) drimane, and 8β(H) homodrimane. In Fig. 5, peaks 3 and 5 respectively refer to 4, 4, 8, 8, 9 and 4, 4, 8, 9, 9-pentamethyl naphthalenes, which is due to the rearrangement alkanes methyl position in the drimane. During the maturation stage, 8α(H) drimane was converted to 8β(H) drimane, high levels of rearrangement drimane often mean highly thermal evolution. Pregnane and homopregnane are highly resistant to degradation, and usually have higher con-tents in highly mature hydrocarbons. In addition the high contents of C30 rearrangement hopanes also re-flect the high maturity of source rocks. Other maturity indicator parameters, such as C29 sterane 20S/

320 Chin.J.Geochem.(2012)31:315–322

(20S+20R) lie between 0.37 and 0.52, and the ratios of ββ/(ββ+aa) range from 0.31 to 0.49, close to the balance end. The abundance of alkyl phenanthrene series in the aromatics is highest, followed by naph-thalene series and fluorene series. The relative abun-dance of PAHs is very similar, mainly because the high maturity led to a convergence of biomarkers.

4 Oil-source correlations

Compact sands of HT-3 in the Longtan Forma-tion were fractured to produce oil flow at about 1.26 m3/d in the HQ area during oil well re-examination. The crude oils are of good quality, with 78.3% satu-rated hydrocarbons and 16.5% aromatics. As shown in Fig. 6a, the carbon numbers of n-alkanes lie between nC12 and nC35. The ratio of C21

-/C22+ is 1.03, (C21+

C22)/(C28+C29) is 4.94. The number of low-carbon saturated hydrocarbons showed a single peak with the former peak type. Pr/Ph value of 1.14 indicates a weak pristane advantage. And CPI and OEP values are 1.01 and 0.98, with no odd-even predominance. In satu-rated hydrocarbons, there is almost no β-carotane al-kyl, but normal salinity, high abundance of tricyclic terpanes, increasing contents of C20, C21 and C23 tri-cyclic terpanes, and a certain amount of long-chain tricyclic terpene alkyl generally reflect the geochemi-cal characteristics of marine organic matter. There are abundant pregnane and homopregnane, with the value of (pregnane+homopregnane)/20R C29 sterane being 0.66, and the ratio of C29 sterane ββ/(ββ+aa) being 0.4, and the methylphenanthrene index was calculated to be R0 1.15, suggesting high maturity. Compared with those of the source rocks in different formations of the HQ area, most of the geochemical parameters are too close to distinguish in the Longtan Formation, e.g. the relative contents of steranes are highly similar, and the Ts/Tm value of oil is 1.16, meanwhile its average value of the Longtan Formation is 1.18. Furthermore, the distributions of fluorine, oxygen fluorine, and sul-fur fluorine are very close to those in the Longtan Formation, so the oil from Well HT-3 is most likely to have come from source rocks of the Longtan Forma-tion. Cretaceous sandstone of Well Rong-2 in the Gecun Formation got low-yielding oil flow of 0.25 m3/d in the oil well re-examination. The contents of saturated hydrocarbons are up to 75.7% and aromat-ics, 18.1%. As shown in Fig. 6, the carbon number distribution of n-alkanes is nC14−nC32, peak carbon nC17; C21

-/C22+ value 3.69, and (C21+C22)/(C28+C29)

5.14. The CPI and OEP are 1.02 and 1, respectively, indicative of no odd-even predominance. The contents of tricyclic terpanes are rather high, with the distribu-tion range of C19-C29, and those of C23 tricyclic ter-pane are even higher than those of C30 hopane, show-ing that the source rocks came from a marine envi-

ronment. The abundance of pregnane and homopreg-nane is high. The (pregnane+homopregnane)/20R C29 sterane value is 3.27. Because of low C29 steranes, 20S/(20S+20R) index and ββ/(ββ+aa) index are 0.46 and 0.57, respectively. The error is large, even more than the balance endpoint, so they cannot be trusted as reliable references. Converted by the methylphenan-threne index, the maturity level is 1.08, showing high crude-oil maturity. While oil samples from Well Jup-ing-1 were collected from the Triassic limestone of the Qinglong Formation, with 56% saturated hydrocar-bons and 26% aromatics. The carbon numbers of n-alkanes range from nC14 to nC33. Peak carbon is nC17. C21

-/C22+ value is 2.67, and (C21+C22)/(C28+C29)

is 6.42. The abundance of low-carbon saturated hy-drocarbons is high. Pr/Ph value of 0.85 is indicative of weak phytane advantage, with no odd-even predomi-nance. The contents of tricyclic terpanes are high. The carbon number distribution of tricyclic terpanes is very similar to that in Well Rong-2, and (pregnane+ homopregnane)/20R C29 sterane value is 1.95. The calculated R0 of methylphenanthrene index is 0.97. These two oil samples are very similar in geochemical characteristics, suggesting that they were collected from the same source rocks. Compared with the dif-ferent Permian and Lower Triassic source rocks in the JR area, most biomarker parameters are much close to those of the source rocks in the Qinglong Formation. and ααα20RC27, C28 and C29 sterane carbon number distribution is L-shaped. The contents and distribution of C20 to C24 tricyclic terpanes are very similar. The carbon isotope values of Wells Rong-2 and Juping-1 are -27.9‰ and -29‰, respectively. The carbon iso-tope distribution of different components is also close to that of the Qinglong Formation. Meanwhile, it shows significant differences from other formations, so the crude oils from Wells Rong-2 and Juping-1 were derived from the Lower Triassic Qinglong For-mation argillaceous source rocks.

5 Geological significance

Geochemical characteristics of the Permian and Lower Triassic source rocks in the lower Yangtze re-gion show their relatively high maturity and impact on the abundance and types of organic matter. For the abundance of organic matter, except the contents of residual total organic carbon, the other parameters of organic matter abundance are degraded to several de-grees, especially the chloroform bitumen "A" parame-ter. It tends to be humified in the types of organic matter. A large number of bituminous minerals were produced during the process of hydrocarbon generat-ing, indicating that the Permian and Lower Triassic source rocks are relatively well developed and have abundant organic matter.

Chin.J.Geochem.(2012)31:315–322 321

Fig. 5. GC-MS showing m/z=123 dicyclic sesquiterpenoids series mass chromatogram. 1, 2. C14 dicyclic sesquiterpanes; 3, 4, 5, 6. C15

dicyclic sesquiterpanes; 7. 8β(H) drimane; 8. 8α(H) drimane; 9, 10. C16 dicyclic sesquiterpanes; 11. 8α(H) homodrimane.

Fig. 6. The biomarker characteristics of low-yielding crude oils in the Lower Yangtze region. a. Well HT-3, Longtan Formation of the Per-

mian in the HQ area; b. Well Rong-2, Gecun Formation of the Cretaceous in the JR area; c. Well Juping-1, Qinglong Formation of the Tri-

assic in the JR area.

From the viewpoint of biomarkers, the sedimen-tary environment is of reducing marine facies, but there may be input of terrestrial organic matter. The biomarkers show non-parity advantages, the high abundance of tricyclic terpenes with long-chain al-kanes and aromatics in phenanthrene and sulfur fluo-rine distribution. The contents of pregnane, homo-pregnane, rearranged hopanes and other compounds are higher at the high maturity stage. A linear rela-tionship is noticed between the methylphenanthrene index and R0. Crude oil conversion R0 is about 1, and that of source rocks is mainly around 1.2 or so, re-flecting a corresponding relationship between the source rocks and crude oils in maturity. By oil-source

correlation, crude oils in the HQ region mainly came from the Permian Longtan Formation; those in the JR region came from the Triassic Qinglong Formation. And the main reservoir is composed of compact sand-stone of the Longtan Formation and fractured carbon-ate rocks of the Qinglong Formation, with the charac-teristics of self-generation and self-storage. Hydro-carbons migrated over short distance, probably be-cause of poor transport conditions or short transport time. While the results of microscopic research showed that a lot of oil and gas generated is left be-hind in the source rocks, suggesting late generation time of current oil flow and favorable preserving en-vironment in the context of strong tectonic movements

322 Chin.J.Geochem.(2012)31:315–322

(Yuan Yusong et al., 2005). Oil and gas migrated up-wards through cracks and even gathered in the Creta-ceous strata on a large scale. Source rocks have great potential for later accumulation and good exploration prospects.

6 Conclusions

(1) Lithologically, the Permian and Lower Trias-sic source rocks are mainly carbonate rocks and mud-stone and then transformed back to carbonate rocks, The potential of carbonate rocks of the Qixia Forma-tion is poor. And in the Qinglong Formation, maybe the muddy layers are the hydrocarbon-generating lay-ers. But in the Gufeng, Longtan and Dalong forma-tions, source rocks are overall productive.

(2) The Permian and Lower Triassic source rocks now are in mature or highly mature stages, It is mi-croscopically shown that the source rocks are in the process of hydrocarbon generating. The contents of residual total organic carbon and the relative contents of extracted components can be regarded as effective indicators to evaluate the abundance of organic matter and its types in the highly mature stage.

(3) Biomarkers show that source rocks and crude oils are from the marine environment. High maturity may lead to a higher abundance of degradation- resistante compounds. Methylphenanthrene parame-ters show that crude oils are generated in the peak phase of source rocks, having similar vitrinite reflec-tance to that of the current source rocks.

(4) Crude oils from Well HT-3 are mainly de-rived from the Permian Longtan Formation, those from Wells Rong-2 and Juping-1 in the JR region are mainly derived from the Lower Triassic Qinglong Formation. Source rocks from different layers have a great potential, mainly for late hydrocarbon genera-tion. All this indicates great exploration prospects.

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Documents

Geochemistry and geological significance of the Upper Paleozoic and Mesozoic source rocks in the Lower Yangtze region