Differential enrichment of oil and gas in carbonate karst reservoir on northern slope of Tazhong uplift, Tarim Basin

The lower Ordovician karst reservoirs on the north slope of Tazhong uplift are basically those of lower porosity and small seepage, and the major reserving spaces of the reservoirs are corrosion holes and caves as well as fissures led by faults. The reservoirs are featured as horizontally continuous layered but vertically separated layered. The high quality reserving-layers are overlapped layer by layer and distributed in the vertical vadose zone and horizontal undercurrent zones between the depths of 0~200 m below the unconformity. Karst reservoirs are of the characteristics such as great in area, containing oil and gas in multiple reserving sections and appearing “oil to the west and gas to the east, oil at the center and gas along the surrounding area” in plane view as a whole. As the cap rock layer, the section 3 to section 5 dense limestone layers of the Yingshan Formation are of the characteristics as “thick in the east and north, but thin in the west and south “in plane view, and appearing “blocky distribution, linked with each other horizontally and superimposed vertically “in the section. Superimposition of multiple high-resistive layers in the Yingshan Formation well configured with the underlying oil and gas reserving layers, which leads to the formation of one or more set of reservoirs and cap-layers that controlling the enrichment of oil and gas. Several tectonic blocks can be divided with boundaries formed by a series of NE strike-slip faults that developed on the north slope of the Tazhong uplift. The oil, gas and water separate normally with each other, i.e. Gas and oil enriched at higher place and water at lower place. And the oil and gas mainly enriched within the scope of 0.5~4.0 km from the major strike-slip fault in the interior of the blocks.