SEDIMENTARY CHARACTERISTICS AND DISTRIBUTION PATTERN OF LACUSTRINE ORGANIC-RICH SHALE IN ORDOS BASIN

In Ordos basin, lacustrine organic-rich shale is mainly located within Chang 7 Member, Yanchang Fm in Upper Triassic system. As the most important source rock, Chang 7 Member is the important target for tight oil exploration. Abundant data, i.e., core - thin section observation, XRD, geochemistry, TOC logging are used to map the lithofacies and sedimentary facies, which helps to understand the composition, texture and organic matter richness of the shale. Moreover, the key factors and distribution patterns of organic-rich shale are discussed in the end.

1. The fine-grained sedimentary system is mapped to understand the lithofacies and sedimentary facies. Black mudstones from deep lacustrine facies dominate in the lower Chang 7 Member, and the delta is located at the northeast part of basin. The shale area is smaller in the middle Chang 7 Member, indicating that deep lacustrine facies shrinks. The delta is located at the northeast and southwest part of the basin, which contributes extensive debris flow to the central lake. In the Upper Chang 7 Member, fine-grained sandstone dominates, which suggests that the deep lacustrine facies shrinks again. The deltas at the northeast and southwest part of basin expand and cause the debris flow to spread.

2. The relationship among petrology, rock texture and organic matters is proclaimed. The organic-rich shale is characterized by the ternary structure. The terrestrial minerals, clay and organic matters compose the ternary structure. TOC is 4-12%, with maximum value 20%. Geochemical and TOC logging analysis suggest that TOC change is of vertical cyclicity. Moreover, the tuff is related to the abundance of OM. During early Chang 7, the volcanic ash favors the bloom of plankton.

3. The transgression-water stratification model for lacustrine organic-rich shale is established. Chang 7 Member is deposited during transgressive period. The lake area was over 5×10⁴km². The water depth can be up to 150m, with salinity<0.1%. We think lakecurrent, water depth and anaerobic environment are the key factors for organic-rich shale distribution. Shale with sapropelic kerogen dominates in the deep lake. Silty mudstone with humus-sapropelic kerogen dominates in semi-deep lake because the water is turbulent with active lakecurrent and some terrestrial debris and plant fragment get involved.