Sequence Stratigraphy And Characteristics Of The Tight Reservoirsin The 3^rd Member Of The Eocene Shahejie Formation, Shulu Sag

With the rapid development of horizontal drilling and hydraulic fracturing, there have been great breakthroughs in unconventional hydrocarbon exploration, including shale gas and tight oil. However, most of the unconventional reservoirs are fine-grained shales and little has been done on terrigenous carbonate rocks. Shulu Sag is an Eocene half-graben basin located in south Jizhong Depression, Bohai Bay Basin and there are 7 production wells within the 3rd Member of the Eocene Shahejie Formation, which consists mainly of terrigenous carbonate rocks.The lower part of 3rd Member of the Shahejie Formation is characterized by thick terrigenous calcirudites and calcilutites with little pore spaces and facies change very rapidly due to the relatively small area of the sag. The lower part of the 3rd member was previously regarded as a third-order sequence using the low-quality seismic profiles, with little information regarding the facies distribution available. All these problems prove to be a major hurdle for further exploration. To solve these problems, we first use cores, thin sections and X-ray diffraction data to develop a new lithofacies classification scheme; secondly, all available data have been integrated to build the sequence stratigraphic framework; at last, the discovered tight oil reservoirs have been analyzed to summarize the controlling factors of tight oil accumulation and suggestion regarding next phase of exploration has been made. Three conclusions have been drawn through the study:1. Five groups of lithologies have been recognized in the study area: rudstones, arenites, siltstones, calcilutites and shales. Rudstone could be further divided into 4 types based on their supporting mechanism and origin of the gravels, namely the grain/matrixsupported terrigenous rudstone and the grain/matrix-supported mixed-source rudstone; arenites and siltstones are mainly composed of carbonates; calcilutites could be divided based on their structures, i.e. laminated and massive calcilutites; shales contain varying amount of carbonates.2. Five depositional sequences have been recognized in the lower part of 3rd Member of the Shahejie Formation through the integration of 3D seismic data, well logs, cores and geochemical data. The 2nd, 3rd, and 4th oil-bearing formations are interpreted to be separate third-order sequences and 1st oil-bearing formation has been divided into two sequences. Lowstand systems tracts(LST) are typically composed of coarse-grained conglomerates; transgressive systems tracts(TST) are characterized by laminated and massive calcilutites; highstand systems tracts(HST) consist of both laminated calcilutites and interbeds of siltstones and arenites.3. Analyses of the tight oil pools within the 3rd Member of Shahejie Formation in Shulu Sag have shown that the accumulations are distributed unevenly both horizontally and vertically. Oil-source rock correlations indicate that the source rocks may act as reservoirs as well and most of the oil has migrated for a short distance. The quality of source rocks and tectonics have major controls over oil accumulation. The best source rocks typically occur near the maximum flooding surface and fractures in reservoirs are controlled by tectonics. The next target should take both source rock and fractures into account.