168 / 2022-02-27 22:22:03

Architectural model of shoal water delta-to-beach bar system -A case study of Es2 Formation, Shengtuo Oilfield，Dongying sag

Shoal water delta; Beach bar; Shengtuo oilfield; Architecture model

Both shoal water delta and beach bar are important oil and gas reservoirs in eastern China’s petroliferous basins. Large-scale oil and gas reservoirs in shoal water delta have been found in Ordos and Songliao basins and many studies on shoal water delta are conducted. The beach bar sand could lead to good reservoirs such as Es₄ formation in Bohai Bay basin. However, there are few studies on the coexistence of shoal water delta and beach bar. Main controlling factors of spatial evolution between these two sandbodies are still unclear. Thus, this study integrates core data, well logging data, dense well net data from Es₂ formation in Shengtuo Oilfield, Dongying sag to investigate sandbody architectural model and controlling factors of shoal water delta-to-beach bar system.

Results show that (1) the sedimentary facies are characterized by shoal water delta and beach bar sedimentary features. The shoal water delta contains distributary channel, mouth bar and overflow. The beach bar includes high energy beach bar and low energy bar. Those microfacies are recognized by their sedimentary structures, well logging response and sandbodies’ thickness differences. (2) Three sand distribution patterns of shoal water delta-to-beach bar system are observed. The continuously branched and striped sandbodies usually consist of distributary channel and overflow. The sand scale of distributary channel varies from 50m in width, 3-5m thick to 450m in width, 3-7m thick. There are three types of distributary channel stacking patterns, lateral splicing-vertical cutted, lateral splicing-vertical superposed and isolated pattern. The isolated lenticular sandbodies mainly parallel to lake shoreline and the sand scale varies a lot from 50m in width, 75m in length, 1-2m thick to 300m in width, 1000m in length, 2-6m thick. Those features indicate beach bar. The semi-continuous irregular shaped sand consists of distributary channel, overflow and mouth bar. Vertically, Distributary channel shallowly incises mouth bar and the thin overflow is located at both sides of distributary channel. (3) An architectural model of shoal water delta-to-beach bar is set up. The distributary channel is branched or continuous in the form of tree-like branches along provenance direction with narrow strip-shaped overflow sandbodies located at both sides. The mouth bar was cut through by distributary channel. In front of mouth bar, the isolated lenticular beach bar sand embedded in lacustrine mudstone. (4) The evolution of distributary channel sand and beach bar sand was controlled by Lake level fluctuation. In the period of lake level drop, the distributary channel progressed and could spread to the whole area, forming widely connected distributary channel sandbodies. In lake level rise period, the distributary channel regressed and disappeared finally, leading to widely distributed beach bar sandbodies in the whole area.