Study On Reservoir Characteristics Of Eocene In Southwest Of Chaluhe Fault Depression, Yitong Basin

Located in the north part of Yitong basin, Chaluhe fault depression which trends NE is faulted by Chaluhe fault in southeast, Daheishan fault in northwest, No.2 fault in southwest, and is bounded by Wulajie slope in northeast. The region of interest, which is about 400km2, lies in the southweatern of Chaluhe fault, and consists of Liangjia structural zone, Xinanbao sag, and the local part of Wanchang percline. In this paper, the target layer is Eocene reservoir, which mostly develops in the 2nd and 3rd members of Yongji Formation and the 1st member of Sheling Formation. Meanwhile, several sets of good oil source rock develop in the area, which supply abundant oil, and act as good regional caprock. In recent years, with the increasing of degree of exploration, commercial oil and gas flow is obtained in several exploration wells located in the research area, so it is the exploration block with the best prospect in Chaluhe fault depression.Up to now, the systematic study of reservoir characteristics and influential factors hasn't done yet, so it has an effect to the further exploration and exploitation. In view of the above problems, guided by reservoir sedimentology and reservoir geology, based on geological background and petroleum geological features, and combining sedimentary facies studied by the predecessors, the author analyses the distribution and petrologic characteristics of Eocene reservoir sand bodies in southwest of Chaluhe fault depression. Furthermore, adopting the reservoir testing techniques, under the methods below, thin-section identify, scanning electron microscope observation, test of mercury intrusion and so on, the author studies reservoir diagenesis, types of reservoir space, distribution and anisotropism of reservoir petrophysical property in detail. Meanwhile, the author analyses the influential factors of reservoir petrophysical property in the aspects of reservoir deposition and diagenesis. This provides responsible geologyical foundation for reasonably classifying layer series of development, selecting inject and production system, and improving the effect of oilfield exploration, and finally increases benefit of exploration and exploitation.Under the methods below, thin-section identify, casting thin-section observation, scanning electron microscope observation, test of mercury intrusion and so on, several conclusions are obtained.â‘ The studied area's sandstone is mainly composed with arkose and lithic arkose, in which the content of quartz is low, feldspar and debris are high, the interstitial material is mainly clay minerals and carbonate cement, and the compositional and textural maturity is low.â‘¡The reservoir diagenesis includes compaction, pressolution, pressure-solution, cementation, replacement and dissolution. The reservoir of Shuangyang Formation is in late diagenesis stage-B. The reservoir of Sheling Formation developed in the center of sag is in late diagenesis stage-B, and the reservoir developed in the margin of sag is in late diagenesis stage-A1. The reservoir of Yongji Formation developed in the center of sag is in late diagenesis stage-A2, and the reservoir developed in the margin of sag is in late diagenesis stage-A1.â‘¢There are five types of porosity which are intergranular porosity, intra-filling porosity, intergranular and intragranular dissolution porosity, and fracture porosity. Among them, the main types are intergranular porosity and intergranular and intragranular dissolution porosity which exist in reservoir at controlled pattern. Complete intergranular porosity dominates over the 3rd member of Yongji Formation. Complete intergranular porosity and intergranular dissolution porosity dominate over the 2nd member of Yongji Formation. Intergranular dissolution porosity dominates over the 1st member of Sheling Formation. There are two types of throat, which are thin throat and subtle throat, and the main is thin throat.â‘£Based on the clascification criteria of pore structure (Dishixiang, 1991), four types of pore structure can be identified, that isâ… B,â…¡A,â…¡B, andâ…¢A, and the main areâ…¡A andâ…¡B.â… B andâ…¡A dominate over the 3rd member of Yongji Formation.â…¡A dominates over the 2nd member of Yongji Formation, andâ…¡B exsists too.â…¡B dominates over the 1st member of Sheling Formation, andâ…¢A exsists only in local area. The types of pore structure is different in different sedimentary facies. In middle fan of sublacustrine fan, pore structure is favorable, andâ… B andâ…¡A are the main types. In the main groove of inner fan, pore structure is unfavorable, andâ…¡B is the main types.By analysing the routine petrophysical data in research area, the reservoir petrophysical property of the 2nd and 3rd members of Yongji Formation is the best favorable with medium porosity and permeability or medium porosity and low permeability, the reservoir petrophysical property of the 1st member of Sheling Formation is the better favorable with low-ultra low porosity and ultro low-very low permeability, and the reservoir petrophysical property of Shuangyang Formation is not very favorable with ultra low-very low porosity and ultro low-very low permeability, partly is even impermeable. The comprehensive study indicates: the reservoir in this area has strong intraformational and interformational nonuniformity, and planar nonuniformity is also obvious, which diminishs from Liangjia structural zone and Wanchang percline to the center of basin with different change degree in different members.Comprehensively analyzing above microscopic and macroscopic characteristics, it is known that the influential factors of reservoir petrophysical property include mainly sedimentation and diagenesis. The influence at the side of deposition is that,on the tabular surface the position where sublacustrine fan sandbody is growing very well whose physical property is more favourabal relatively,and in middle fan of sublacustrine fan, pore structure,porosity, permeability are more favourable than inner or outer fan . dissolution is favorable which improves reservoir petrophysical property,and buildup three high bands of porosity and permeability at 2000m-2300m,2500m-3000m and 3700m,which provide a good location for the hydrocarbon accumulation.