Seismic prediction of source rock distribution in the Lower Member of Pinghu Formation in the Pingbei area of Xihu Sag

Drilling wells are rarely and unevenly distributed in the Pingbei area of the Xihu sag and sample analysis data are also lacking. However, the heterogeneity of source rocks is very strong in different layers, which has made it unclear the distribution pattern of source rocks. In order to solve the problem, this paper studied the characteristics of the organic facies and the thickness distribution patterns of the source rocks in the Lower Member of Pinghu Formation taking the Pingbei area as a case, based on the integration of geochemical, logging and seismic data. The method of seismic facies-sedimentary facies-organic facies transformation and the method of seismic velocity and lithology analysis are adopted for prediction of favorable source rocks. The results show that: (1) The organic facies of source rocks in the Lower Member of the Pinghu Formation can be classified into three facies types, i.e. tidal delta front A, intertidal zone B and confined sea C. The tidal delta front A facies, located in the center of the study area, is the most developed one in the Second Member of Lower Pinghu Formation, while the intertidal B facies is the most developed in the First Member of Lower Pinghu Formation, which is mainly located in the western region of the study area. (2) The thickness distribution pattern of the source rocks in the Lower Pinghu Formation are clarified. The source rocks in the Second Member of Lower Pinghu Formation are zonated from east to west, thinner in the west and thicker in the east, and the maximum thickness may be over 500 meters; while the source rocks in the First Member of Lower Pinghu Formation are generally thin in the northwest and thick in the southeast, with a maximum thickness more than 1000 meters in the southeast; (3) The integrated evaluation and analysis of source rocks suggest that the favorable source rocks in the Second Member of Lower Pinghu Formation are mainly distributed in the southeast of the study area, and the favorable source rocks in the First Member of Lower Pinghu Formation are mainly distributed in the middle east of the study area. The research results are significant to the early prediction and evaluation of source rocks in the study area and other depressions.