| The research on hydrocarbon accumulation mechanism and enrichment rules is a key fundamental work in scientifically predicting reservoir distribution, conducting well placement, exploring and developing of reservoirs. As one of important hydrocarbon accumulation belts in petroleumliferious basins, the gentle slope belt of a half graben basin with large extension area, various tectonic structures, and sedimentary sandbody changing fast, complicated hydrocarbon enrichment and accumulation rules is an important hydrocarbon accumulation belt. After many years exploration, reservoirs easy to be explored have been all found. There is still a large number of remaining resources. In order to make a new breakthrough in exploring oil and excavating the remaining resources potential, it’s necessary to research the accumulation mechanism and enrichment rules of hydrocarbon.The study object of this paper is the western gentle slope of Shulu sag. According to the result of the third resource evaluation, the amount of resource in Shulu west slope is up to 51.2Million ton(mt), the proved reserve of this area can be reached to 14.6308mt, and the remaining resource is to 36.56mt, resource converting rate is merely 28.58%. The main restraining factors are as following: the concrete space extension of source rock with generating hydrocarbon potential, the main controlling mechanism of fault on reservoirs and migration styles have not be studied clearly, the main control factors of accumulation and enrich rules of hydrocarbon need to be further studied. So studying some essential factors such as structural, sedimentary, source rock and hydrocarbon migration characteristics of the gentle slope belt of Shulu sag by using multi-disciplinary theories and techniques related to geology, sedimentology, geophysics, sequence stratigraphy and analytical geochemistry, by referring to petroleum system theory, and by basing on geology, seismic, logging, drilling, geochemistry and reservoir developmental dynamic, this paper is focusing on main controlling factors and accumulation rules of Shahejie Formation rich in hydrocarbon. The achieved accomplishments and recognition are as following:(1)It is the first time in Shulu sag to determine the space extension of Es3 of marl source rock by using series of well logging curve. Based on the geochemical data and combined with seismic data, the computing model of generating hydrocarbon quantity used to evaluate the source rock hydrocarbon generating potential is set up, and point out that the extension of the source rock in space is the main controlling factor of hydrocarbon enrichment.(2)The Shulu slope is characterized by dipping in the eastern part, lifting in the western part, being steep at early stage but gentle at the late stage. The steep and narrow slope at the early stage is the main controlling factor forming the unconformity reservoirs and conglomerate reservoirs. The wide and gentle slope forming at the late stage rendered the change of sandbody to become complicated and river channel sandbody being distributed along the footwall of the antithetic fault, affecting reservoir distribution.(3)The hydrocarbon migration system controls hydrocarbon accumulation area. By studying the hydrocarbon migration mechanism: in the intra-belt of the slope, the primary migration direction is vertical, while lateral migration ability is relatively weak, and in the extra belt of the slope, lateral migration along the unconformity of bottom of Es3 is the main migrating style, the source fault determine the distribution of the fault reservoir.(4)The mechanism of controlling reservoir of antithetic fault is the configuration relationship between the fault distance and sedimentary succession in studying area: antithetic fault control the reservoirs forming in Es2, the consequent fault control the reservoirs forming in Es3. The fault assembling pattern in Es2 in Xicaogu tectonic belt and the extension of sediments determining the enrichment of the hydrocarbon, among these, the thick oil bed is prone to occur in the reverse-arrange fault assembling pattern.(5)Crack and unconformity control and adjust the distribution of reservoir. The weather surface at the top of multi-stage conglomerate fan in the intra-belt and its fracture growth control the distribution of conglomerate reservoir. Unconformity controls the oil accumulation in the extra slope. In the steep tectonic belt in Xicaogu, it depends on the sandbody and unconformity to migrate hydrocarbon to form formation reservoir in carbonate formation under the unconformity.(6)The study area is currently found to be characterized by "source-fault-facies". The rolling development orientation is predicted by using these characteristics and combining the research result of oil accumulation rules. |
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