Sedimentary Facies, Diagenesis And Reservoir Characteristics Of Changxing Formation In Daxian-Xuanhan, Northeast Sichuan

At present, Daxian-Xuanhan area in northeastern Sichuan basin is an important exploratory and exploitative region of Sinopec. And there are open questions about sedimentary facies, microfacies, diagenesis of Changxing Formation in this area. Guided by carbonate reservoir sedimentology & reservoir geology and diagenesis, based on observation of microsection, C/O isotope and fluid inclusion, by means of the combined methods of indoor & outdoor and macroscopy & microscopy, sedimentary facies, microfacies, diagenesis and reservoir characteristics of Changxing Formation are studied in depth. Furthermore, the control on reservoir distribution is discussed and favorable reservoir distribution is predicted. The systemic research is very important to reduce the cost of oil & gas exploration, improve exploration efficiency and gain great economic benefit in this area.Firstly, by means of drilling core, outcrop, rock slice, seismic and well logging data, sedimentary facies and microfacies of target stratum is studied in detail, and depositional model is established and then the plane distribution of sedimentary facies is concluded. The main sedimentary facies types include shoal facies of platform margin, organic reef facies of platform margin, platform evaporate facies, open-platform facies, restricted platform facies and carbonate platform ramp facies, and the front three types of facies are the uppermost facies belt. The main sedimentary microfacies types include subordinate organisms dolomite microfacies, sparry grain(biogenic debris, sand cutting, alga chippings and so on) dolomite microfacies, detention gravel debris dolomite microfacies, boundstone microfacies, non-lamina dolomite mudstone microfacies, lamina dolomite mudstone microfacies, lamina lime mudstone microfacies, sparry grainstone microfacies and testaceous chippings limestone microfacies, and the front three types of microfacies are the uppermost microfacies type.Secondly, on the basis of sedimentary facies and microfacies, diagenesis, diagenetic stage, diagenetic environment and diagenetic sequence is analysed with lots of rock slices and geochemistry data. At the same time, the plane distribution of diagenetic facies is concluded, and then pore evolepment of different microfacies is contrasted. The author reaches a conclusion: the favorable diagenesis is dolomitization, corrosion and recrystallization; the favorable diagenetic environment is exposed-buried corrosion, admixing water-buried corrosion and vapor dolomitization-buried environment. Moreover, reef-shoal dolomite (subordinate organisms and sparry grain dolomite microfacies) reservoir pores develop with a large-scale distribution, abundant dissolved pore types and excellent reservoir capability; evaporite platform dolomite (non-lamina dolomite mudstone microfacies) takes second place with average reservoir capability; reef limestone (boundstone microfacies) pores develop with lower reservoir capability, regarded as III class of reservoir at most; open-platform limestone (non-lamina dolomite mudstone microfacies)pores underdevelop with no reservoir capability.Lastly, sedimentary facies and diagenesis is the most principal controlling factor of favorable reservoir, by means of analysing log, seismic response characteristic and lithology, physical property of the reservoir. Basic sedimentation controls reservoir distribution, critical diagenesis not only influences evolvement of reservoir space and characteristic of pore structure, but also determines current reservoir distribution. Namely, with respect to Changxing Formation in the region of interest, the most favorable facies belt is reef-shoal dolomite facies, the more favorable facies belt is restricted platform facies-platform evaporate facies; the most favorable microfacies belt is subordinate organisms and sparry grain dolomite microfacies, the more favorable microfacies belt is non-lamina dolomite mudstone microfacies.