Study On Sedimentary Facies And Reservoir Distribution Of Changxing Formation In The Southeastern Margin Of Liangjiang - Liangping Trough

As one of the most important gas reservoirs in the Kaijiang-Liangping region, the upper Permian Changxing formation has been a hot concern in domestic, but different views come out on its development and distribution principles. Under the guidance of typical sequence stratigraphy, sedimentology and reservoir geology, and with the Changxing formation in southeastern edge of Kaijiang-Liangping trough as research object, this thesis had preliminarily clarified the reservoirs’ development and distribution regularity about Changxing formation, and also the relationship among sequence stratigraphy, sedimentary facies and reservoirs by an integrating analysis on adjacent outcrops, well logging, thin sections and seismic data.On the basis of lithological associations and sedimentary cycles, Changxing formation can be divided into three members from button to top in the non-reef strata regions(namely P3ch1,P3ch2,ch2and P3ch3), and also make it isochronally extended into the reef strata regions. According to a detailed analysis on well logging and seismic profiles, Changxing formation are composed of two3rd-order sequence attaching to Ⅱ type, with each containing a transgressive system tract(namely TST1, TST2) and a high stand tract(namely HST1, HST2) in the3rd-order sequence stratigraphic framework. Just as a slight clarification, TST1, HST1and TST2-HST2respectively corresponds to P3Ch1, P3ch2and P3ch3, with a good regional comparability.By analyzing drilling cores, thin sections, well logging curve and seismic data, the sedimentary facies(especially reef and shoal facies) spatial and temporal distribution and evolution of Changxing formation had been isochronally revealed in different3rd-order sequence system tract in the study area. At stage of TST1, the carbonate rocks deposited in a land-connecting platform model, which mainly developed open platform facies and few small-scale bioclastic shoals, while platform margin facies belt only come out latterly. At stage of HST1, with great deposits thickening of a quick and massive expanding development of reef and shoal facies, platform margin facies belt matured. Meanwhile, open platform took the predominant role with few bioclastic shoals scattered distributing in the platform. At stage of TST2, inheriting the former depositional setting, platform margin facies belt growed up with a small-scale reefs and bioclastic shoals, and the inner platform model continued with little change. At early stage of HST2, the depositional setting changed with a smaller-scale marginal reefs and bioclastic shoals developing in a limited part of Wubaiti region; but at late stage, tidal flat facies gradually substituted for marginal reefs, and the inner platform evolved into restricted platform.Based on analysis of drilling cores, casting thin sections, well-logging and testing data, the dominant dissolved-pore(pore-cavity) reservoir types of Changing formation varied, mainly including dolomite or limestone reservoir from organic-reef facies, dolomite or limestone reservoir from bioclastic-shoal facies, dolomite reservoir from tidal-flat, reef-flat or shoal-flat facies, of which the dolomite generally owns a better reserving property than the limestone. Vertically, the reservoir usually growed at the later stage of high stand tract, and have a closely relationship with reefs and shoals, mainly distributed in the middle-upper part of a single reef or shoal progradational depositional system with relative sea level shallowing upwards, in which dolomiting and dissolving strongly for reservoirs of high quality. While in a single reef or shoal retrogradational depositional system as relative sea level deepping upwards, the limestone hold the superior with a little dolomiting and dissolving, which generally resulted in a poor reserving property. As for the planar surface, the reservoir often distributed in the platform margin belt and the inner platform, while few come out from slop or trough region. Taking such indexes as reservoir type, degree of dissolving and dolomiting, and numbers and scales of high quality reservoirs into consideration, the study area can be divided into four types of reservoir zone qualitatively, with the marginal reefs-shoals facies district having strongly dissolving and dolomiting as type I zone, the marginal reefs-shoals or the shoals-in-platform facies district having relative strongly dissolving and dolomiting as type II zone, the limestone reservoirs district of marginal or inner-platform shoals facies having weakly dissolving and dolomiting as type Ⅲ zone, while the tight limestone reservoirs district of open-platform or restricted-platform facies as type Ⅳ zone.