The Genesis Of The Feixianguan Formation Dolomite In The Tieshan~Longmen Area, Northeastern Sichuan, And Its Relationship With The Reservoir

The oolitic shoal gas reservoir of Triassic Feixianguan Formation,NE Sichuan Basin,is one of major breakthroughs in carbonate gas exploration in recent years.Under basic conditions of structure and traps,the distribution of the residual oolitic dolomite reservoir with most excellent porosity and permeability determines the oolitic shoal gas reservoirs exploration results directly.Current exploration and development and related theoretical research focus most on Eastern Kaijiang-Liangping Trough.And the vast areas of Western Trough lack relevant research apart from the Yuanba area.Therefore,based on full understanding of stratigraphy,sedimentary facies,rock types and distribution characteristics of the oolitic shoal,mechanism of dolomitization of Feixianguan Formation,Western Kaijiang-Liangping Trough is studied in this paper.Based on macro core features and microscopic features,it is illustrated that the reservoir rock types of oolitic shoal reservoir in the study area consist of dolomite-bearing oolitic limestone and residual oolitic dolomite.The reservoir space of dolomite-bearing oolitic limestone mainly includes intergranular dissolved pores and rarely dolomite intercrystalline pores within oolites caused by selective dolomitization.The porosity and permeability of dolomite-bearing oolitic limestone has smaller range which is 0.68%?8.76%(mean value=2.23%)and<0.0001×10-3?m2-23.3×10-3?m2(mean value=74.3×10-3?m2,N=258)respectively.By comparison,the reservoir spaces of residual oolitic dolomite are intergranular dissolved pores.Its porosity ranges froml.54%to 23.85%(mean value=7 13%)and permeability ranges from 0.01×10-3?m2 to 423×10-3?m2(mean value=74.3×10-3?m2,N=86).The mercury injection data demonstrates that residual oolitic dolomite has higher average porosity than dolomite-bearing oolitic limestone,of which mercury injection curve shows stair-shaped,with coarse distortion,low displacement pressure,high-middle throat radius in features.The sediments of oolitic shoal provide material basis for development of reservoir in Feixianguan Formation,especially in platform margin facies.The dolomitization is the key to formation of high quality residual oolitic dolomite reservoirs.Based on trace elements,carbon and oxygen isotope and rare earth elements,It is considered that the best residual oolitic dolomite in the oolitic beach reservoir is the combination result of the dolomitization of the retrograde seepage and hydrothermal fluids,which is characterized by low Mn,and Fe content(mean value=184×10-6 and 2×10-6),lower ?18O value above normal seawater value,and slightly lower ?13C than normal seawater value.The REE analysis shows that it not only has Ce positive anomaly,but also has Eu positive anomaly.The euhedral dolomite within dolomite-bearing oolitic limestone lower ?13C(0.12‰?0.64 ‰)and ?18O(-8.97‰?-7.77‰)than normal seawater value(?18O=-0.8?-0.5‰,?13C=+2.8?+3.1‰,PDB),significantly low Mn content,and non-cathodoluminescence.It shows the strong dilution effect of meteoric water,which is the product of dolomitization of mixed water,and is not favorable for the formation of high-quality oolitic dolomite reservoirs.Moreover,burial dissolution also provides scale of secondary pores for Feixianguan Formation reservoir,the reservoir developed to provide the necessary conditions.Fractures play an important role in further improving for the permeability of oolitic reservoirs.