Organic-Rich Shale Origin and Shale Oil Prospect in Lacustrine Basin, China

Caineng Zou^1,2,3, Rukai Zhu^1,2,3, Xuanjun Yuan^1,3, Jingwei Cui^1,3, Lan Wang¹, Senhu Lin¹, Zhi Yang¹, Qun Liu¹, and Zhen Qiu^1
1Research Institute of Petroleum Exploration & Development, PetroChina, Beijing, China
²State Key Laboratory of Enhanced Oil Recovery, Beijing, China
³CNPC Key Laboratory of Oil and Gas Reservoirs, Beijing, China

Great successes have been obtained on the exploration and development of shale gas and tight oil globally. The research on the fine-grained sediments in deep/semi-deep lacustrines, i.e., siltstones, limestones and shale has become the new highlight for sedimentology. Black organic-rich shales were recognized as source rocks or seals but not reservoirs in conventional petroleum geology, and they were not the targets for the exploration. The industrial success of shale gas and tight oil in North America make the explorers to pay more attention to the special black shale system. Tight sand oil, tight limestone oil, shale gas and shale oil have become the new choices for unconventional petroleum exploration. As one of the most important unconventional oil resources, shale oil may probably become the important exploration target in the further 20~30 years or even shorter.

Black lacustrine shale in China is widely distributed in space and time framework. Currently, most of them are in oil window and abundant oil and gas showings during the drilling, indicating great potential of shale oil. There are six lacustrine shale systems in China, including Permian, Triassic, Jurassic, Craterous, Paleogene and Neogene system. Permian shale is located in the western & southern depressions of Junggar Basin, with three formations including Fengcheng Fm., Xiazijie Fm. & Wuerhe Fm. Triassic shale is located in central & southern part of Ordos Basin and Chang 7 shale is the most important one. Jurassic shale is located in central, northern and eastern part of Sichuan Basin, and shale in Ziliujing Fm. is the best. Craterous shale is widespread in Songliao Basin, and shales are well developed in Qingshankou Fm. and Nenjiang Fm. Paleogene shale in Bohai Bay Basin are dominated in the first section, third section & fourth section in Shahejie Fm.

Lacustrine shales are formed in stratified water of anoxic environment which is located in the central depressions or sags in basin. Moreover, the water in semi-deep/deep lacustrine basin is relatively quiet and fine-grained sediments accumulate at the bottom of water in vertical way. Several factors could contribute to the preservation and richness of organic matters, including (1) the rapid sediments accumulation resulting from organic matter aggregates which is the production of flocculation; (2) the anoxic environment at the bottom of stratified water; (3) organism bloom caused by nutrient from the intermittent saline intrusion; (4) the rapid settling of clay minerals because of water salinization.

According to the study on the organic-rich shales, i.e., the Upper Triassic Chang 7 shale in Ordos Basin and Craterous Qingshankou shale in Songliao Basin, we find that lacustrine shales are highly heterogeneous in vertical direction. Abundant fossils are discovered in shales, such as ostracoda, conchostracans, bivalve, fish scale and so on. Two typical lamina structures are discovered. The first lamina structure is composed by clay and organic matter, and the second one is composed by silt, clay and organic matter. Moreover, lots of sedimentary structures, i.e., wavy bedding, discontinuous wavy bedding, nervation bedding and flaser bedding are developed in black shales, indicating the bottom currents were active during the formation of shale. Organic matter is distributed along the layers, or rich in some local places, or dispersed in the matrix.

Organic-rich shale in middle/lower part of Chang 7 Fm. is the most pratical section for shale oil. The thickness of shale is 21m~36m, with burial depth 1100m~2900m and the exploration area is 5×10⁴km². Chang 7 shale was deposited in semi-deep/deep lacustrine and the water depth is infered to be 50m~120m based on Co data. Homonemeaes are suggested to be the major organisms from organic macerals analysis and molecule geochemical data. Chang 7 shale is deposited in fresh water environment from a series of data including inorganic B, Sr/Ba and gammacerane suggest. Pr/Ph, V/(V + Ni), Ni /Co data and pyrites occurrence all indicate anoxic environment. The high contents of Mo, Cu, U, V and Pb are possible to be related to organism bloom caused by nutrient from the tuff. In a word, Chang 7 shales is formed in high productivity mode caused by volcanic ashes and is preserved in anoxic environment.

Chang 7 shale oil is of five geological features. (1) Excellent source rocks with great hydrocarbon (HC) generation potential. The basic geochemical information include: Type I and type IIA kerogen, Ro = 0.7% ~ 1.1%, TOC = 1.4%~25.6%, S1 = 1.2~11.6mg/g, Chloroform A = 0.2%~1.2%, and pyrite content = 5.4%~34.5%; HC generation potential of shale is five times of that of mudstone; (2) Nano-pores and parrel bedding fractures with diameter =50~300nm dominate the storage space and a few micrometer-scale pores are distributed locally; (3) The content of brittle minerals, i.e., quartz, feldspar, calcite and dolomite reach 41% in average and the clay mineral content is less than 50%, suggesting high brittleness index; (4) Abnormal low pressure and light oil. The pressure coefficient is 0.7~0.9, and the density & viscosity of oil is 0.75~0.85 g/cm³, 0.7~5mPa•s respectively; (5) The liquid HC is absorbed in the surface and inner OM. In inter-particle pores of pyrites, clay minerals and brittle minerals, the liquid HC can be in absorbed state or in free state. Liquid HC is free in the bedding fractures.

Preliminary assessment of recoverable resources for lacustrine shale oil in China is 3~6 billion tons. A series of technologies, i.e., volumetric fracturing with horizontal wells, natural fractures enlargement, artificial reservoirs via coarse grain injection, shale oil mobility improvement via natural gas injection are probably to become critical to further industrial development of shale oil. In order to support further shale oil development, we should pay more attention to the research on shale oil distribution, to the sweet-spotting, and to pilot reconstruction. Based on the experience from the successful exploration of shale gas and tight oil in North America, shale oil is probably to be firstly broken through and be industrially developed 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