Reservoir characteristics and main control factors of the Carboniferous Huanglong Formation in Sichuan Basin

Carboniferous Huanglong Formation is the main producing formation of gas reservoir in the Sichuan Basin.Through years of exploration and development work,it has entered the stratigraphic-lithologic gas pool exploration stage.Therefore,it is especially important to strengthen the research of the reservoir characteristics and distribution prediction in the study area.Based on abundant core observation and thin section analysis,combined with the SEM,reservoir physics and pore texture research,this paper analyzes the reservoir development characteristics and main control factors and predicts the potential reservoir distribution area.Studies show that Carboniferous Huanglong Formation in the Sichuan Basin developed restricted platform supratidal flat-intertidal flat-subtidal flat;among them,the intertidal flat-subtidal flat shallow shoal microfacies were developed;the dominant lithologic units are grain dolomite(or its remnant),crystalline powder-cryptomere dolomite and dolomitic karst breccias;The noticeable reservoir spaces are intercrystalline pore and intercrystalline dissolved pore,while the secondary pore types are intergranular dissolved pores,intragranular dissolved pores,moldic pores and fractures;the porosity of Huanglong Formation ranges from 0.03% to 30.67%,average 2.59%,while the permeability ranges from 0.001μm2 to 290×10-3μm2,average 1.56×10-3μm2.The reservoir of Huanglong Formation is characterized by low porosity and extremely low permeability,as well as a relative good correlation between porosity and permeability,belonging to crack-pore reservoir;the porethroat assemblages are dominated by small pore and medium-fine throat.The potential reservoir is controlled by lithology,sedimentary facies and late stage diagenesis;the reservoir physics of grain dolomite in shallow shoal microfacies and of crystalline dolomite in tidal flat facies are better;sedimentary microfacies controlled the distribution range and scale of reservoir;deuteric dolomitization is the foundation of reservoir formation,it is characterized by multi-stages and multi-causes;among all of the dolomitization stages,the penecontemporaneous and early diagenetic dolomitization are most beneficial to the formation of potential reservoir;paleokarst events could enlarge the reservoir scales,and its zonation,the fluid hydrodynamic condition and lithology in different dissolution,and microtopography are the direct control factors of karst reservoir spatial distribution patterns and development scales;paleokarst events are the key to improving the quality of reservoir.According to the dolomite thickness map,dolomitic karst breccias map and reservoir porosity map,we chose the contour lines that exceed 3% as a constraint boundary,predicted the lebensraum of potential reservoir in the Sichuan Basin,and among them,we have evaluated 7reservoirs in rank I,and 5in rankⅡ.The results above-mentioned haveimportant effects on the reservoir anticipation and gas prospecting in the Sichuan Basin.