STUDY ON SEDIMENTARY MICROFACIES OF THE 1~(ST) MEMBER OF SHAHEJIE FORMATION AND ITS IMPLICATIONS FOR OIL IN THE WENNAN OILFIELD

The study area of this article,Wennan Oilfield, is located in the southern part of the central uplift belt of the Dongpu Sag,which is of about 224km~2.It is one of the potential petroleum belts and is adjacent to the Qianliyuan Oil-bearing Depression. The 1~ st Member of the Shahejie Formation is the most important development bed series in the Wennan Oilfield. Three types of reservoirs are recognized: lithologic reservoir,lithologic-fault reservoir and fault block reservoir. The distribution of sand-body and fault system plays an important role to oil reservoir cumulating,but the distribution of sandbody is controlled by characteristics of source rocks and sedimentary paleo-topography. A close relationship exists between sedimentary facies and oil reservoirs. The reservoir beds in the west of the southwest slope include the areas of Qiaokou,Dongming,Gegangji and Sancunji districts that are characterized by delta front sub-facies,which could be divided into distributary channel,river-mouth bar and front sheet sand micro-facies. Beds formed in both of distributary channel and river-mouth bar environments are 4-7m in thickness and with defined distribution. Front sheet sand is of 2—3m in thickness and is a principal reservoir bed in the Wennan Oilfield. Oil reservoirs of high product can be formed when structural height are coupled with subaqueous distributary channel or river-mouth bar facies.