The Impacts Of Diagenesis On Carrying Capability Of Unconformity Of Ke-Xia Fault Belt In Junggar Basin

With the higher level of hydrocarbon exploration, unconformity reservoirs have being paid more and more attention and become primary exploration targets in oilfields. Predecessors have done a lot of work about unconformity, but less work about the capability of unconformity. The study about the impacts of diagenesis on carrying capability of unconformity is a new topic in domestic.On the basis of a large number of previous studies, this dissertation studied several characteristics of unconformity, such as the type, the distribution, the spatial structure, the depth, and the lithology combination, according to the observation of the outcrop, the core and the samples and a mass of collected data of seismic, mud logging, geochemical and well logging. There are six types of unconformity in study area: muti-truncation unconformity, truncation unconformity, overlap unconformity, fault-fold unconformity, fold unconformity and parallel unconformity, and unconformity reservoirs mainly distribute in multi-truncation unconformity and truncation unconformity. The unconformity has three parts in vertical from top to bottom: rock above the unconformity, weathered clay and semi-weathered rock. From uplift belt to depression, there are muti-truncation unconformity→fold unconformity, fault-fold unconformity, truncation unconformity→overlap unconformity→parallel unconformity in order, and the thickness of rock above the unconformity become thinner, weathered clay become thicker. The rock above the unconformity has five types of lithology, and the semi-weathered rock has seven. The conformity above the top of Jurassic, Triassic, Permian has six, twenty-two, twenty-eight types of lithology combination separately. And unconformity reservoirs mainly exist in the following four types: AHJ (conglomerate- clay- sandstone), AHK (conglomerate- clay- conglomerate), BHJ (sandstone- clay- sandstone), BHK (sandstone- clay- conglomerate). The unconformity may have two effective transport passages: rock above the unconformity (conglomerate above the unconformity especially) and semi-weathered rock.The evidence of hydrocarbon migration in unconformity, which was showed in logging, inclusions, slices and geochemistry characteristics, indicated that the unconformities were important migration pathways. It was found that the hydrocarbon experienced wide-ranging migrations and cementations three times at least. And the directions of hydrocarbon migration, which were from Mabei oilfied to Wuxia district and from Mabei oilfied to Kebai district, can be shown from the characteristics of the geochemistry.Based on the unconformity characteristic research, making fine anatomy for four typical unconformity reservoirs, which locate in Wellblock Zhong32, Wellblock Xia9, Mabei Oilfield, Wellblock Ke75. The transportation system of unconformity reservoirs performs as compounded type of fault and unconformity mostly in study area.The rock above conformity, the original physical characteristics of which are good, is beneficial to form overlap reservoirs. And the semi-weathered rock, which has more dissolution pores, is propitious to form unconformity-screened reservoirs. The distribution of unconformity reservoirs are mainly controlled by"two lines and one structure", ie., overlap line controls the distribution of overlap reservoirs in the rock above unconformity, pinch-out line controls the distribution of unconformity-screened reservoirs in semi-weathered rock, the development ability and sealing ability of weathered clay are crucial to the formation of unconformity reservoirs.Through the observation of 115 common thin sections and 6 scanning electron microscope samples and the analysis of previous test result of lots of thin sections, scanning electron microscope and X-diffraction, diagenetic sequences and diagenetic stages of semi-weathered rock in the four unconformity above are divided. And diagenetic characteristics were summarised, which are distinct. It has a lot of dissolution pores and more cracks. Rock's alteration characteristics are widespread and regular, which become stronger between different unconformities and become weaker in the same uncomformity with increasing depth.Based on the study of the characteristics of unconformity and diagenesis, the impacts of diagenesis on carrying capability of unconformity were studied by qualitative evaluation and quantitative description. The main diagenesis, which influence the physical character of the reservoirs, are compaction, cementation, dissolution and fracturing. Compaction and cementation make pores decrease, dissolution and fracturing make pores increase. Using apparent compaction rate, apparent cementation rate and apparent dissolution rate to measure compaction, cementation and dissolution separately as quantitative parameters, then determined the classification boundaries and distinguished various levels for different diagenesis according to statistics. The sequences about the effect level that the four kinds of diagenesis are different in distinct areas, which is compaction> cementation> dissolution> fracturing in Wellblock Zhong32, compaction> dissolution> cementation> fracturing in Wellblock Xia9 and Wellblock Ke75, compaction> dissolution= cementation> fracturing in Mabei Oilfield. In addition, weathered clay has enough sealing capability in stable region and a deeper depth, which would reach the standard to seal hydrocarbon, as breakthrough pressure of weathered clay increases exponentially with depth.Finally, based on the study of unconformity characteristics of distribution, hydrocarbon accumulation and diagenesis, predicting Mabei slope area is the favorable zone of unconformity reservoirs, which located in the junction among Wuerhe Oilfield, Xiazijie Oilfield and Mabei Oilfield.