The Study On The Efficiency Of Hydrocarbon Migration Through Unconformity In Faulted Basin

In recent years, hydrocarbon exploration demonstrates that unconformity reservoirs play an important role in newly found reservoirs and become primary exploration targets in faulted basins, eastern China. As the main hydrocarbon pathway, one of the objectives for unconformity study is to prove how hydrocarbon to migrate through unconformity from source rocks to reservoirs, i.e., the efficiency of hydrocarbon migration through unconformity which directly control the formation of unconformity reservoirs. However, few studies have been done on migration efficiency through unconformity, which restricts the exploration and development of unconformity reservoirs. Therefore, further studies should be carried on, which is significant for the formation and occurrence of unconformity reservoirs.This dissertation focuses on Jiyang and Jizhong Depressions, Bohai Bay basin. Based on the unconformity structure and the types of migration passages, it discusses the heterogeneity and physical modeling experiments of unconformity. Moreover, the efficiency of hydrocarbon migration through unconformity is intensively studied and prospects of unconformity reservoirs are predicted.Study shows that the vertical structure of unconformity in faulted basin includes three parts: basal conglomerate, weathered clay and semi-weathered rock, among which basal conglomerate and semi-weathered rock are major passages for hydrocarbon migration, and semi-weathered rocks are the main reservoirs for petroleum accumulation. The structure of unconformity can be subdivided into 48 types. If weathered clay is absent and the overlying layer is sandstone, the unconformity mainly becomes carrier bed, and the sealing or trap is formed if the overlying is mudstone. When weathered clay develops, the unconformity tends to trap oil and gas.Two kinds of unconformity migration passages are presented, which are dual migration passage consisting of two types of highly effective transport passages and single migration passage, repectively. The dual migration passage occurs between semi-weathered rock and basal conglomerate. The cavern-fracture system of semi-weathered rock and the connected pores of basal conglomerate can be regarded as the major passages for hydrocarbon migration. They connected faults and sand bodies formed in different periods to construct the regional migrating network which controlled the hydrocarbon migration.The unconformity structure displays obvious heterogeneity because of the differences in geometry, continuity, thickness, lithology and physical property. The unconformity shows strong interformational, intraformational and plane heterogeneity. The semi-weathered layer is more heterogeneous than the basal conglomerate layer. With the consideration of permeability mutation coefficient, permeability dash coefficient and permeability graded differences, the evaluation criterion of unconformity heterogeneity has been established and hydrocarbon can migrate and accumulate through semi-weathered layer when Vk≥0.7.Modeling experiments are designed to simulate the process of hydrocarbon migration through unconformity. The experiment results can be summarized as follows:①Unconformity can be a good migration passage, and hydrocarbon migrates mainly along carrier beds with better physical property.②Most hydrocarbon migrates along the semi-weathered layer.③Hydrocarbon migration through unconformity is characterized by an obvious interformational, intraformational and plane heterogeneity.④Opened faults, which connect the semi-weathered layer and basal conglomerate layer, not only can be a good passage, but also improve the transportation capability of unconformity.⑤The physical properties of unconformity control the hydrocarbon migration. Hydrocarbon has faster migration velocity, larger volume and longer migration distance in semi-weathered layer than in basal conglomerate layer.⑥Under the same experimental conditions, the semi-weathered layer has the highest oil saturation and the largest loss. Moreover, the semi-weathered layer has broader effective passage volume than the basal conglomerate layer and shows stronger transportation capability.Based on the preceding research and physical modeling experiments, this paper discusses the efficiency of hydrocarbon migration through unconformity in faulted basins:①The lateral distance of hydrocarbon migration through unconformity can reach 50km, and favourable range is from 5km to 20km.②Different depressions have different migration rates, from 0.28km/Ma to 8.33km/Ma.③Hydrocarbon migrates from sag to uplift laterally, and migrates along ridges not gullies vertically.④Semi-weathered layer and basal conglomerate layer are always the migration passages in geological time, and cavern-fracture system of semi-weathered layer has a stronger transportation capability than the connected pores of basal conglomerate layer. Therefore, hydrocarbon migration and accumulation occurred mainly in semi-weathered rocks.⑤A lot of factors influence hydrocarbon migration through unconformity, including oil-source, migration drive force, unconformity features (such as regional distribution, structural nature, migration passages and heterogeneity) and the collocation of unconformity and other migration pathways. Among these, oil-source and migration drive force are prerequisite, and unconformity features and the collocation of unconformity and other migration pathways are key factors.In the end, favourable regions of unconformity reservoirs in faulted basins are predicted:①The stratigraphic onlap and denudation zones in basins or depressions are the favorable belt to discover unconformity reservoirs.②The stratigraphic onlap and denudation areas combine with nose structure belts to form a prolific zone of unconformity reservoirs.③The inner uplifts in basins or depressions will be the important targets for unconformity reservoirs (especially buried hill reservoirs) in the future.