Genesis of the Upper Ordovician dolomite in southern and peripheral Sichuan Basin
The industrial gas flow tested recently in Well Weihan 1 in the porous dolomite of the Upper Ordovician Baota Formation in the Sichuan Basin marks a significant breakthrough in the exploration of Ordovician natural gas.So far,however,the genesis of this set of dolomite has not been understood yet,which hinders its subsequent exploration and development.In this paper,the data of old wells are reviewed,and the outcrop data in the 1∶200000 regional survey map is sorted out again.In addition,microscopic observation and carbon/oxygen stable isotope and in-situ micro area trace element analysis and test are performed on cuttings,cores and outcrops samples.Finally,the petrologic characteristics,spatial distribution patterns,and geochemical characteristics of the dolomite are clarified.Based on this,the genesis model of the Upper Ordovician dolomite in the southern and peripheral Sichuan Basin is established.The following results are obtained.First,the Upper Ordovician in the study area is mainly composed of fine to medium-crystalline dolomite,powder to fine-crystalline dolomite,and limy dolomite.The former is mainly developed in the Upper Ordovician Daqing Formation outside the basin,while the latter two are primarily developed in the Daqing Formation and the Baota Formation in the Weiyuan area inside the basin.The powder to fine-crystalline dolomite of the Baota Formation constitutes a layered porous reservoir with highly automorphous crystalline grains and developed inter-crystalline pores.Second,the Upper Ordovician dolomite in the study area is characterized by light rare earth element depletion,heavy rare earth element enrichment,positive La anomaly,negative Ce anomaly,and positive Y anomaly,which reflects a marine origin of the dolomitized fluids.Additionally,the dolomite's oxygen isotopes are significantly heavier than normal seawater deposits,and gypsum pseudomorph is developed and distributed in the upper parts of upward shallowing sequences and the restricted environments such as tidal flats and sags,which suggests that dolomitized fluids may be evaporated seawater,and their infiltration from top to bottom to dolomitize the underlying strata is the main pathway for the formation of dolomite.Third,two types of dolomite genesis are identified in the study area,i.e.,"tidal flat brine-sourced type"and"lagoon brine-sourced type".The former refers to dolomite formed due to the infiltration of heavy brine water between pores in evaporative tidal flat sediments,such as the Baota Formation dolomite in the Weiyuan area inside the basin.The latter refers to dolomite formed by the infiltration of heavy brine water accumulated in the lower parts of restricted lagoons,such as the Daqing Formation dolomite in the Yuexi-Yiliang area outside the basin.In conclusion,there may be tidal flat dolomite developed in the Baota Formation at the northern slope of Central Sichuan Paleouplift,which is expected to be an important target of risk exploration in the next step.
万方数据
维普
