Abstract:

Taking the Lingshui Formation of the Paleogene in the Qiongdongnan Basin as an example, the problems associated with oil and gas exploration include limited drilling data, complex seismic data, development of fault system, and difficulty predicting favorable exploration targets. In this study, under the guidance of the classical sequence stratigraphy theory of Vail, we established the isochronal stratigraphic framework by closely combining the shallow water drilling data with the deep water seismic data in delineating the target strata into four three-level sequences. Evidences from rock types, sedimentary structures, authigenic minerals and biological fossils showed that the target strata formed in a marine sedimentary environment. In the isochronous stratigraphic framework, we carried out the single element analysis for each geogenetic pathway system through sediment analysis, fault system interpretation, and unconformity characterization. The pathway system of the target strata was genetically divided into three main types-reservoir, fracture and unconformity, and we described in detail the spatial developmental characteristics of different types of transport systems. The slope and pelvic floor fans in the lower system domain, the fan delta, beach bar and shore sand in the pathway system domain, and the fan delta and braided river delta in the higher system domain are the main sedimentary facies types corresponding to the reservoir transport system. The large controlling concave fault spreading in the northeast can be a favorable channel for oil and gas migration. The unconformity is divided into four categories: parallel, overlap, truncation and overlap-truncation unconformities. The widely developed unconformity in the sequence interface can be a favorable migration channel and plays an important role in oil/gas migration and transmission. We further analyzed the spatial combinations of different pathway systems and summed up the modes of combination into six types: fault+unconformity (reservoir) or “T” type transport, fault+unconformity (reservoir) or comb type transport, fault+reservoir or grid type transport, fault+unconformity or stepped type transport, and fracture type transport (fracture+bedding surface+pore). We then located each type of pathway system in the basin, and established the pathway system model. Based on the analysis of petroleum geological characteristics, such as temperature/pressure field and fluid potential as well as hydrocarbon source rock and cap rock, we predicted the favorable oil and gas exploration zones, which mainly included the north margin of the Yanan Depression, the Songxi Depression, the north margin of the Songnan Depression, the periphery of the North Reef Depression, the south slope of the Lingshui Depression, and the north and south margins of the Changchang Depression. Among the four sequences, sequence Ⅰ was the most favorable zone, followed by sequences Ⅲ and IV, whereas sequence Ⅱ was the least favorable zone.

Key words: Qiongdongnan Basin, Lingshui Formation, pathway system, seismic facies, reservoir, fault, unconformity, combination model of pathway system