Study On Depositional Facies And Petroleum Play Prediction Of The First Member Of Shahejie Formation In Shenxian Depression, Jizhong Subbasin

Shenxian depression was located in the south part of Jizhong subbasin, which was a nonmanne faulted basin controlled by two contemporaneous faults and whose rift system was complicated. The resources conversion of Shenxian depression was low and the exploration potential was high. Shenxian depression had a long exploration history, but the pre-research target was mainly focused on the structural trap. Hence, it was significant to carry on researches on subtle traps for making a breakthrough in the petroleum exploration in Shenxian depression.Lacus intrusion water intrusion come at the early stage of the first member of Shahejie formation, which not only formed important oil producing formation in the area but also made good conditions for subtle traps development.On the basis of observation and description of core, combining the analysis of the regional geologic settings,applying the geology and geophysics then, following the research path of point(well) -line(profile) -plane(planar map) -tract(temporal evolution and special distribution)- point(potential sand bodies), the sedimentary system's planar distribution and subtle traps of the first member of Shehejie formation in Shenxian depression were studied and the potential zones were predicated in this thesis. Through the above researches, the achievements of this thesis were as follows:1. The first member of Shahejie formation was divided into two long-term cycles (third-order sequence) by the application of the high-resolution sequence stratigraphy and recognition of the Shahejie formation sequence boundary. Isochronic chronostratic framework was established by the isochronic strata correlation on the long-term cyclic sequence scale, which indicated that tectonic activity was severe and formation thickness changed quickly at the early stage of the first member of Shahejie formation, while at the late stage, tectonic activity was feeble and formation thickness changed little.2. Four types of sedimentary facies that were delta,subsea apron,beach bar and lake were identified on the basis of systematic core observation and description, using grain size analysis which can reflect hydrodynamic force and combined with facies sequence and well logging data.3.On the basis of single well sedimentary facies, guided by sedimentation model, combined with palaeogeomorphology,seismic attribution and tectonic feature, sedimentary system's planar distribution maps were described on the long-term cyclic sequence scale.On plane map, many subsea aprons were developed along steep bank with characteristics of "small and many"; braided stream delta was development along gentle slope and its distribution range was vast. In vertical direction, distribution of sedimentary facies was influenced by lake water obviously. At the early stage of the first Member of Shahejie formation lacus water intrusion was obvious and delta presented the characteristics of "big front and small plain"; while at the late stage of the first Member of Shahejie formation lacus water subsided and delta presented the characteristic of "small front and big plain".4. Two types of lithologic traps that were lithologic updip pinchout trap and sand lens trap, two types of stratigraphic traps that were denudation unconformity trap and overlap unconformity trap and structural lithologic trap were developed in the study area.The development of lithologic traps was controlled chiefly by break belt and stratigraphic traps chiefly by unconformity. Structural lithologic traps were developed in ShenNan anticline. The favorable area of subtle traps was optimized following the "superposition" path and pointed out an exploratory target in the west flank of ShenNan anticline.