Oil Accumulation Mechanism Of Fault Traps And Petroliferous Prediction

Based on full use of geology, seismic, logging and analysis of various data, and combining with regional geological background, this paper makes systematic analysis on structural characteristics, formation and evolution of fractures in the two reservoirs enrichment -Panyu 4 sag and Huizhou sag in Pearl River Mouth basin, and then divides the fracture systems. Through the study on petroliferous features and oil-water distribution of fault traps, firstly, this paper analyses mechanism of reservoir forming and ability of hydrocarbon accumulation, secondly, it summarizes the main factors controlling oil accumulation of fault traps, and finally, it makes evaluation and optimization on the target traps. We can get conclusions on studies as follows:In profile, Pearl River Mouth basin consists of three tectonospheres, including basement structure, downfaulted basin structure and depression basin structure. The formation and evolution of fractures has experienced four tectonic deformation, respectively are Wenchang Formation, Enping Formation, Zhuhai-Zhujiang～Hanjiang Formation and Yuehai Formation～Quaternary period. Four sets of fault system have developed in Pearl River Mouth basin, that is, the extensional faults in early stage (typeⅠ), the weak extensional faults in metaphase (typeⅡ), the tense-shearing faults in later period (typeⅢ) and faults which are early extensional and late tense-shearing (typeⅠ-Ⅲ). The target layers of Zhujiang Formation consist of three types of fault system, namely, typeⅡ, typeⅢand typeⅠ-Ⅲ.Moreover, traps are mainly formed during the period of Yuehai Formation, and often distributed in the local compression areas of fault deformation, including terminal area of a single fault (typeⅠ), left-stepping bending part of a single fault (typeⅡ), strata bridge areas with left-stepping distribution of two or more faults (typeⅢ) and cross sites when two or more faults converge(typeⅣ). There are mainly typeⅡandⅢfault traps which are all rich in oil in Panyu 4 sag, while in Huizhou sag, there are mainly typeⅡ,ⅢandⅣfault traps among which typeⅣones are relatively the best. The petroliferous ability of fault traps is under the control of charging conditions, firstly, if the range of fault traps inside the main hydrocarbon-generating sub sags and outside the source rock is less than 5km, hydrocarbon supplying ability of source rock is better, moreover, according to different warping directions of strata on two walls of fault, the way of hydrocarbon injecting into traps are divided into two types, i.e., vertically upward injection which is favorable to hydrocarbon accumulation and vertically downward injection which is adverse, and the way determines the difficulty degree of hydrocarbon injecting vertically into reservoir along fractures. It is the lateral sealing ability of faults controlling traps that determines hydrocarbon accumulation ability of fault traps, and in the region, this paper has divided lateral sealing into fault rock sealing and lithology juxtaposition sealing, there, the sealing way of Trap PY5-1 and PY11-5(T35-T41) in Panyu 4 sag belongs to lithology juxtaposition sealing type, while the others are types of fault rock sealing. However, due to influence of thin multilateral sand and shale on Trap PY5-1 and PYll-5(T35-T41), the hydrocarbon column height of juxtaposition sealing is shorter, so, whether traps can gather much hydrocarbon or not depends on the size of anticline amplitude. And when referring to traps of fault rock sealing, firstly, this paper uses SGR in fault zones to evaluate the sealing ability, and then checks it with difference of displacement pressure between fault zone and reservoir, finally, it proves that faults in the study area are all with certain sealing ability, however, the predicted hydrocarbon column height is generally shorter than trap amplitude, this phenomenon indicates that the filling extent of traps is closely related to the lateral sealing ability of faults. The predicted hydrocarbon column height is all taller than that of nowadays, that is, the actual hydrocarbon column height has not reached that of lateral sealing, and from this, we can conclude that fault lateral sealing ability is not only affected by SGR in fault zones but also by late damage degree of fault rocks. Through the analyzing the causes of loss traps in the study areas, it is considered that the hydrocarbon supplying ability of source rock, the difficulty degree of hydrocarbon injection, the lateral sealing ability of faults and the damage degree of faults are the main controlling factors of hydrocarbon accumulation in the region. And on the basis of evaluating and optimizing target fault traps, it is considered that only when the value of comprehensive evaluation is greater than 3.0, can fault traps become successful ones and have conditions of hydrocarbon accumulation, in the region, Trap PY11-9, PY4-4 and PY10-5 in Panyu 4 sag, Trap XJ24-1S and HZ 19-6 in Huizhou sag are the successful ones, and they are the preferred target traps for the next drilling step.