--> Abstract: The Sedimentary Characteristics and Distribution Pattern of Lacustrine Organic-Rich Shale in Ordos Basin, by Xuanjun Yuan, Senhu Lin, Qun Liu, and Hao Guo; #90180 (2013)

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The Sedimentary Characteristics and Distribution Pattern of Lacustrine Organic-Rich Shale in Ordos Basin

Xuanjun Yuan, Senhu Lin, Qun Liu, and Hao Guo
Research Institute of Petroleum Exploration & Development (RIPED), PetroChina, Beijing, China

In Ordos basin, lacustrine organic-rich shale is mainly located within the Chang 7 Member of upper Triassic Yanchang Formation. Chang 7 Member is the most important source rock in the basin, and now is the important target for tight oil exploration. The Chang 7 Member can be divided into 3 submembers. Based on the industrial mapping lithofacies and sedimentary facies, we carried out multiple analyses, such as core - thin section observation, XRD, geochemistry, TOC logging and so on. These analyses help us to understand the composition, texture and organic matter richness of the shale. We finally discuss the controlling factors and distribution patterns of organic-rich shale.

1. The fine-grained sedimentary system of the 3 submembers are mapped. The lithofacies and sedimentary facies are clarified. We built a database that includes about 300 wells drilling report and made a statistics for 10 rock types, including shale, mudstone, silty mudstone, muddy siltstone and fine sandstone. Then we work out the lithofacies and sedimentary facies maps. In the lower Chang 7 Member, deep lacustrine facies dominates. It can be up to 5x104km2. The delta is located at the Northeast basin. In the middle Chang 7 Member, mudstone and fine sandstone dominate. The shale area gets smaller, which indicates that the deep lacustrine facies shrinks. The delta is located at the Northeast and Southwest basin, which causes the extensive debris flow to distribute in the central lake. In the upper Chang 7 Member, the fine sandstone dominates which suggests that the deep lacustrine facies shrinks again. The delta at the Northeast and Southwest basin expands and causes the debris flow to spread.

2. The relationship between the composition and TOC, and the relationship between texture and organic matter is proclaimed. The shale is mainly comprised of clay minerals, quartz and feldspar, as well a few kerogen, pyrite and calcite. The thin sections indicate rich lamination in the shale which is characterized by the ternary structure. The terrestrial minerals, clay and organic matters compose the ternary structure. The TOC of the shale is 4-12%, can be up to 20%. The geochemistry and TOC logging analyses suggest that the TOC change has vertical cyclicity. What’s more, the tuff is related to the abundance of organic matters. During early Chang 7, the volcanic ash favors the bloom of plankton.

3. We establish the sedimentary model of lacustrine organic-rich shale. It is a kind of transgression stratification model. The stage of Chang 7 Member deposition is the most important transgression period. The lake area was over 10x104km2 at that time. The depth of lake can be up to 200m. The water was fresh with paleo-salinity less than 0.1%. During the transgression period, the water depth got bigger soon, which cause the water stratification due to temperature variation, to generate a large-scale anaerobic environment which favors the formation of organic-rich shale. We think that the lake current, water depth and anaerobic environment is the main control factor for organic-rich shale distribution. The abyss gets small influence from lake current, is so quiet. The dead plankton is the main source of organic matters in shale. Therefore, in the deep lake, the shale dominates and characterized by sapropelic kerogen. In the semi-deep lake near the delta front, lake current affects more. The water is turbulent. Some terrestrial debris and plant fragment get involved. So the silty mudstone dominates and characterized by humus-sapropelic kerogen.

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