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Study on the Fine-grained Sediments and Tight Light Oil of Chang 7 Formation in Upper Triassic, Ordos Basin, China

Lin, Senhu; Zou, Cai-neng; Yuan, Xuanjun

Unconventional petroleum, especially the tight light oil and shale gas is becoming the strategic replacement of global petroleum resources. A lot of exploration practice has demonstrated that the tight light oil is one type of the continuous unconventional oil trapped in the source rock formation with low permeability less than 1 mD. As the tight light oil is mainly located in the center of basin where is almost consisted of fine-grained sediments, we did an exhaustive study on the characteristics, development and origin of the fine-grained sediments of Chang 7 Formation of upper Triassic, Ordos Basin, China.

Five rock types are dominated in the study area, including very fine sandstone, argillaceous siltstone, silty mudstone, mudstone and shale. The thickness of these five rocks is about 97% of the strata. These fine-grained sediments distribute following a rule: the shale only exist in the deepest area and shrink obviously with the migration of the depocenter; the sandstone have stable scope in Northeast and Southwest due to delta construction, and expand in the central basin because of the frequent sandy debris flow; the argillaceous siltstone mainly distribute ashore, have a large scale in the central basin as well; the silty mudstone are concentrated at the offshore of lake; and the boundary of mudstone is essentially coincident with middle-deep lacustrine areas.

Since each microfacies has an extraordinary lithology composition in thickness, the difference between these compositions can be a useful indicator for sedimentary facies. Based on core observation and statistic analysis, we build the thickness composition of each microfacies with these five rock types. It is ‘sedimentary codes'. By overlapping the thickness percentage isolines maps of five rock types, quantitative mapping of sedimentary facies comes true.

Combining with the reservoir estimate, it is concluded that 3 factors--shale faces, sandy debris flow faces and reservoir rock type, control the distribution of tight light oil. The overlapped area of shale faces and sandy debris flow faces is the sweet spot. In the sweet spot, the region rich in arkose has better porosity and permeability. By this means, we predicted 5 favorable tight light oil plays. The total area is 12650 km2 with 6.94×108t oil reserves.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013