| The Shulu Sag is located in the southwestern corner of the Jizhong Depression, Bohai Bay Basin. The lower part of the Shahejie 3 Formation(Es3L) in the middle and southern of the Shulu Sag developed a carbonate minerals-rich rock sequence, mainly rudstone and marlstone. The rudstone and marlstone sequence in the Shulu Sag is a tight oil play and has become an important exploration target for the Petro China Huabei Oilfield. Nevertheless, the characteristics and control factors of this tight reservoir is still not clear. In this thesis, based on analysis of drilled cores, thin sections, scanning electron microscopy, logging data, as well as a verity of test data, an intensive study of lithofacies, organic geochemical and the reservoirs characteristics is conducted. The major controlling factors of qualities of this reservoirs are identified, and further more the reservoir evaluation is documented.The main minerals in the study lithology include calcite, dolomite, clay minerals, quartz, feldspar and pyrite, and calcite is the predominant mineral. Four origins of calcite in the study area are suggested as following: 1) allochthonous origin, 2) biochemical origin, 3) diagenesis origin, 4) bioclastic origin. Considering the compositions, particle sizes, and structures etc., the lithfacies of the study area is divided into eight categories. They are clast-supported terrigenous rudstones, Matrix-supported terrigenous rudstones, mixed-sources rudstones, carbonate lithic sandstone, carbonate lithic siltstone, laminated marlstone, massive marlstone, lime mudstone.The Es3 L organic matter-rich marlstone in the Shulu sag has large thickness, wide distribution, and contains large amounts of thermally mature Type II and Type I kerogen deposited under profundal lake conditions. It has great original hydrocarbon generative potential. The main organic facies in the study include organic facies AB, B, BC, C, CD and D. The quality of organic matter closed to the edge of the lake basin is inferior, which are mainly organic face D and C. Towards to the central of the lake basin, the quality of organic matter becomes better, which are mainly organic face B and AB. Different sequences develop different organic facies. And the best quality of organic matter is from SQ2, which develops thick organic facies AB. The results of oil-source correlation indicates that crude oil of Es3 L in the Shulu Sag was coming from the Es3 L organic-rich marlstone. It reveals that the tight oil system in Es3 L of the Shulu Sag is a type of special self-generating and self-preserving, accumulation near the source, continuous petroleum accumulation system.This tight reservoir mainly develops micron- and nano-scale pores. The reservoir space types include pores and fractures in the gravels, seams around gravels, intergranular pores, intercrystalline pores, intragranular pores, dissolution pores, organic matter pores, fractures, etc. Pores and fractures in the gravels, seams around gravels, dolomite intercrystalline pores, dissolution pores, organic pores and fractures are the relatively high-quality reservoir space types among them. The quality of reservoir is mainly controlled by lithofacies, organic facies, and fractures. The organic matter not only controls hydrocarbon generation capacity, but also the development of dissolution porosity, organic matter porosity, abnormal pressure fractures etc. Among the different lithofacies, clast-supported terrigenous rudstones and organic matter-rich laminated marlstone have the best reservoir properties. Combined with the characteristics of organic facies, lithofacies, fractures, and accumulation system, the potential exploration target are thought to be distributed in specific sequence, zone, and district. In conclusion, the potential exploration target in the Shulu Sag is distributed in SQ1 and SQ2, the west slope break zone, and the "C" shaped around the subsag zone. |
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