New insight into oil and gas exploration in Miocene and Late Oligocene strata in Qiongdongnan basin

Abstract

Oil and gas exploration prospect for the Miocene and Late Oligocene strata in Qiongdongnan (琼东南) basin are evaluated. The structural-sedimentary and reservoir characteristics are discussed and helpful conclusions are drawn. It was proved that there are mainly two reservoir and capping assemblages in Qiongdongnan basin by drilled wells, i.e., one assemblage of the second and the third members of Lingshui (陵水) Formation, upper Sanya (三亚) and Meishan (梅山) formations. According to hydrocarbon evaluation from burial history and well data, all source rocks in Yacheng (崖城) and Lingshui formations have reached the mature-overmature stage. High temperature and overpressure provide dynamic conditions for oil and gas migration, while the long-term activity of basin controlling faults is adverse to their accumulation in upheaved areas. However, because the secondary faults in the basin ceased their activities at the end of the Sanya Period, and the Sanya Formation was later covered by the regional overlying layer of the Meishan Formation, the blocks and low uplifts near the secondary faults were favorable for oil and gas accumulation. Furthermore, as another important pointed area for gas migration, the highest position of a structural ridge should reasonably be a range that is covered by marine mudstone or sandy mudstone during the whole subsiding period, and therefore, the reservoir bed should not be in that position but at its inferior eminence or adjacent slope zone. Therefore, we can draw a conclusion that the inferior low uplifts and structural ridges rather than the upheavals or the top of a structural ridge, probably, have huge reservoirs, and should be chief exploration targets in the Qiongdongnan basin.

Keywords

Qiongdongnan basin / Miocene / Late Oligocene / structural ridge / low uplift

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