Formation Mechanism And Development Model Of The Longwangmiao Formation Carbonate Shoal Reservoir In The Sichuan Basin

In 2012,since the discovery of the Anyue gas field and high-quality carbonate shoal reservoir in the Longwangmiao Formation,central Sichuan Basin.However,in the past10 years,dolo-grainstone reservoirs with different reservoir properties have been discovered in the different positions of Sichuan Basin.In order to improve the development mechanism of the Longwangmiao Formation and the distribution prediction of favorable zones,the key problems to be solved are to reveal the distribution law and controlling factors of dolo-grainstone reservoir,especially the genetic type of carbonate shoal,dolomitization mechanism and pore reduction effect of pressure dissolution and cementation during burial.This study uses core,outcrop,well logging and seismic data,sedimentary petrology,sequence stratigraphy,and rock geochemistry analysis testing technology.Around the development and formation of carbonate shoal reservoir,four conclusions have been obtained.First,the ramp depositional environment of the Longwangmiao Formation is divided into seven microfacies,five subfacies types.Three types of depositional cycles were identified in order to clarify the development and distribution of shoals.The planar distribution of depositional cycles is controlled by paleogeomorphology.The overlapping of multi-stage shoals,shoal-tidal flat,and intershoal-shoal cycles are mainly developed in the central Sichuan uplift,the broad and gentle uplift of northern Sichuan Basin,and the local-paleobathymetric highs of eastern and south-eastern Sichuan Basin,respectively.The vertical distribution of depositional cycle is controlled by sea-level fluctuations,and overlapping of multi-stage shoals and shoal-tidal flat were more likely than intershoalshoal to develop in the upper part of the 4th order sequence.Second,five rock types associated with dolomitization have been identified.The spatial and temporal distribution of dolostone is controlled by the paleogeomorphology and the sea level fluctuations.The dolo-grainstone is mainly distributed in the central Sichuan uplift and the central part of northern Sichuan Basin.The 4th order sequence,located in the middle and upper part of the 3rd order sequence,has a high dolomitization degree.The dolomitization fluid in the Longwangmiao Formation is seawater with a certain degree of evaporation,and three penecontemporaneous dolomitization models are established.Third,the compaction of carbonate shoal in the Longwangmiao Formation is weak,and the reservoir pore reduction is mainly caused by pressure solution and cementation.The pressure dissolution is affected by rock types and their mineral composition and textural components,and it has been found that the pure dolo-grainstone has the strongest resistance to pressure dissolution.The pore reduction models of pressure solution-cementation in the burial process of five carbonate shoal reservoirs are established,and found that the pore loss due to pressure solution-cementation is the lowest in the overlapping of multi-stage shoals,with an average pore loss of 4.0%.Four,the reservoir rocks of the Longwangmiao Formation include(residual)dolo-grainstone and medium-fine crystalline dolostone.The reservoir space types mainly include intergranular(dissolved)pores,intercrystalline(dissolved)pores,intragranular dissolved pores and karst caves,with intergranular(dissolved)pores as the main ones.The development and distribution of reservoir are controlled by deposition and diagenesis.Strong energy shoal deposition and quasisyngenetic exposure controlled by paleogeomorphology and sea level changes are the basis of reservoir,and compaction,pressure solution,and cementation are the key to the preservation of deep reservoirs.In summary,the dolo-grainstone reservoirs of the Longwangmiao Formation are mainly developed in the central Sichuan uplift and the northern Sichuan Basin.Vertically,it mainly develops in the upper part of the sequence.Three types of carbonate shoal reservoir development models were established: the reservoir of overlapping of multistage shoal reservoirs in the central Sichuan Basin have the highest content of dolograinstone,strong pressure dissolution and cementation resistance,and the average porosity is 4.7%.The shoal-tidal flat reservoirs in the northern Sichuan Basin have high content of dolo-grainstone and strong compressive dissolution and cementation resistance,and currently has an average porosity of 3.13%.The intershoal-shoal reservoirs in the east and southeast Sichuan Basin are low in content of dolo-grainstone,with an average porosity of 1.74% at present.