四川盆地南缘下志留统龙马溪组页岩气储层矿物成分特征及意义

Characteristics and significance of mineral compositions of Lower Silurian Longmaxi Formation shale gas reservoir in the southern margin of Sichuan Basin

运用X射线衍射技术,对四川盆地南缘长宁—兴文地区下志留统龙马溪组39个露头泥岩/页岩样品进行了定性和定量分析。结果表明,龙马溪组泥岩/页岩矿物成分以黏土矿物和石英为主,平均含量分别为53.39%和29.15%;其次为方解石,平均含量为5.46%;另外,还含有少量的长石、白云石、石膏、黄铁矿等矿物。龙马溪组为浅海陆棚沉积,对有机质的富集和保存有利,为页岩气藏的形成提供了良好的沉积条件;黏土矿物组合特征反映出龙马溪组已经历晚成岩阶段,对应有机质演化的成熟—高成熟阶段,具有适宜页岩气形成的成熟度条件。黏土矿物对页岩气藏的形成和开发有一定积极意义,龙马溪组底部厚度约为30m的地层,伊利石和绿泥石含量低、相应孔隙率高且脆性矿物石英含量大于50%,是理想的页岩气重点勘探开发层位。页岩气储层矿物成分研究为其吸附储存、裂缝评价、渗流运移、压裂造缝和工艺性能等的研究提供了坚实的基础。

Qualitative and quantitative analyses by X-ray diffraction（XRD） were performed on 39 outcrop shale samples from the Lower Silurian Longmaxi Formation in the southern margin of the Sichuan Basin（the Changning-Xingwen area）,the results showed that major components of the Longmaxi Formation mud shale are composed of clay minerals,quartz and calcite,with an average of 53.39%,29.15% and 5.46%,respectively.In addition,other mineral components include feldspar,dolomite,gypsum,and pyrite,with an average less than 5% for each mineral.The Longmaxi Formation consists of neritic shelf deposits that favor the enrichment and preservation of organic matter and provide a good depositional condition for the formation of shale gas reservoirs.Clay mineral compositions characterize a late diagenetic stage experienced by the Longmaxi Formation,which corresponds to a mature or highly mature evolutionary stage of hydrocarbons and is provided with a suitable maturity condition for shale gas formation.Moreover,clay minerals are also favorable for the formation and development of shale gas reservoirs,especially the basal interval of the Longmaxi Formation that is 30m thick and contains low illite and chlorite,correspondingly with high porosity and more than 50% of quartz,suggesting an ideal horizon for shale-gas exploration and development.Therefore,studies on mineral compositions of shale-gas reservoirs will provide a substantial basis for examining the adsorption and storage of shale gas as well as the fracture evaluation,seepage flow migration,width generation by fracturing and technological functions in shale gas exploration and development.