Sedimentary Environment and Lithofacies Characteristics of Black Shales in Ordos Basin

Lan Wang, Chun Yang, and Tong Lin
Research Institute of Petroleum Exploration and Development (RIPED), PetroChina, Beijing, China

Black shale is studied as source rock in petroleum geology. With deep water exploration for and production of shale gas and oil becoming progressively more extensive and successful, it clearly important to understand the Sedimentary environment, processes of depositon and organic matter preservation of black shale source rocks in the deep water recently. Black shales are defined as generally fine-grained sediments or sedimentary rocks that contain> 1% total organic carbon (TOC). The formation of black shales involved water depth, sedimentary rate, provenance and preservation of organic matter.

Black shales of Yanchang formation Chang 7 in Ordos basin formed in deep lake with thickness of 100m and area of 5x10⁴km². It contain 13.75% average total organic carbon (TOC) and OM type is I and II1. Chang 7 black shales composed of oil shale, silt-rich mudstones and a few lamina siltstone and sandstone. It can be heterogeneous on a range of spatial scales. Chang 7 black shales comprise a variable mixture of clay minerals (e.g., illite, mixed layer illite-smectite, kaolinite, chlorite), quartz, feldspars, carbonates, amorphous material, and organic matter. rLamina geometries vary from being continuous parallel and wavy to nonparallel and curved. lenticular bedding and flaser bedding can observed. Dark kerogen-rich laminae and silt-rich laminae composed couplets. Opaque pyrite framboids are scattered throughout the dark kerogen-rich laminae and increased with TOC. Geochemical indicators suggest Chang 7 black shales deposited under very restricted conditions of brackish lake. It is researched the brackish lake is related saline intrusion. Salinization of lake water not only added the rate of deposition, but also benefit the preserve of OM. It generally considered that black shales deposed in persistent anoxic bottom-water and low-energy suspension setting. A variety of sedimentary structures in Chang 7 black shales including compacted ripples and erosional bases indicate the environments were more dynamic. Wave-enhanced sedimentary gravity flow and storm-induced currents may be two important Transport mechanism. It can increase high sedimentation rate so that OM can be buried quickly. OM can remove from the upper oxygenated part of the sediment column.

The research of Chang 7 black shales indicated that we should address the lithofacies, distribution and depositional processes of fine-grained sediments. Particular pay attention to the type of bottom currents and influence to sediments.

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