--> --> Abstract: Organic Geochemical Characteristics of Fine-Grained Sedimentary Rocks in China, by Shuichang Zhang, Ningning Zhong, Jianping Chen, and Bin Zhang; #90180 (2013)

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Organic Geochemical Characteristics of Fine-Grained Sedimentary Rocks in China

Shuichang Zhang1, Ningning Zhong2, Jianping Chen1, and Bin Zhang1
1Research Institute of Petroleum Exploration and Development, PetroChina, Beijing, China
2China University of Petroleum, Beijing, China

Hydrocarbon source rocks in sedimentary basins in China were influenced by unique tectonic and palaeogeographic and palaeoclimatic settings and are of great varieties and types. Recent exploration proves that marine sediments, deep to semi-deep lacustrine sediments, even swamp sediments, can become oil-generation rocks with huge reserves.

The marine source rocks in China occurred mainly in the early Paleozoic, Cambrian, Ordovician and Silurian strata. They are the most common source rocks in the lower Palaeozoic system, occurring in open marine basin-facies, slope-facies and isolated basin-facies. They are mostly silica shale, with organic carbon content of around 3~8% and occasionally up to 17~22%. The source rocks are dominated by Type II organic matter, sourcing from low aquatic organisms. Due to their old ages and prolonged thermal history, the Ro value (the organic maturity index) can reach up to 2.5~3.5%, reaching an over-mature stage. The majority of hydrocarbon discovered in the Sichuan Basin is dry gas cracked from crude oils. For example, (1) the natural gas in the Cambrian reservoirs of the Weiyuan and Gaoshiti-Moxi gas fields, were derived from the lower Cambrian oil cracking; (2) gas in the east Sichuan and northeast Sichuan gas fields were sourced from the Silurian oil cracking, with partial contribution from the Permian marine source rock. The Cambrian and Silurian dark shale with abundant organic matters are the major target intervals of shale gas in the Sichuan Basin. The Cambrian and Ordovician source rocks of the Tarim Basin, occurring in different places resulted in different products. The Cambrian source rocks occurred in the basin-facies, and the maturity is quite high due to their deep burial, becoming the major gas source for the basin. The middle-upper Ordovician source rocks, occurring in the slope-facies with relatively low maturity, become the major oil source. The oil and gas generated from these two sets of source rocks, mixed in the uplifts and formed large amount of condensate oil.

Lacustrine source rocks form the key source for oil accumulation in the sedimentary basins in China. The main source rocks contains: Permian (Jungar Basin), Triassic (Ordos Basin), Cretaceous (Songliao, Erlian and Jiuquan Basins) and Eocene (Bohaiwan, South China Sea Basins). The major sedimentary settings are semi-deep to deep humid depressions. The organic matter content is around 2~5%, and HI can reach 600~800 mg/g TOC. The organic matter is mainly of Type I or Type II, with algal biomass being the major sources. The Ro values of these source rocks are between 0.7 and 1.3%, within the “oil window”. Therefore the product is mainly of oil. The oils in the huge oilfields or oilfield groups, like the Songliao and Bohaiwan Basins, all came from lacustrine shale. Besides, in some places oil can also be found in the shale system for its moderate maturity, e.g., the Triassic shale of the Ordos Basin and the Permian of the Jungaer Basin.

The common characteristics of source rock in China are that they have abundant of organic matter, favorable organic types, which are crucial for large-scale oil and gas accumulations. However, the maturity of source rocks from marine and lacustrine sediments are polarized due to different development layers. The Type I or II lacustrine source rocks are currently in the “oil window” (Ro = 0.7~1.3%) and mainly produces oil. The Type II marine source rocks are in an over-mature stage and (Ro > 2.0%) and produces gas. These characteristics may indicate that abundant oil and gas accumulations, both convention and unconventional, can be found in China.

AAPG Datapages/Search and Discovery Article #90180©AAPG/SEPM/China University of Petroleum/PetroChina-RIPED Joint Research Conference, Beijing, China, September 23-28, 2013