尽管地热能的发展前景广阔,但随着地热能利用的优势逐渐显现,国内各大型企业已纷纷投身到新能源建设热潮中去,行业竞争日趋激烈.随着地热开发日渐火爆,各地政府纷纷出台规范性政策,对地热开发的手续、技术、管理等多方面提出更高要求,开发政策日趋严格.
The exploration for the Middle Permian Qixia Formation,an important successor to the Cambrian producing formations,has made great breakthroughs in recent years in the Gaoshiti-Moxi area of central Sichuan Basin.Based on core thin-section observation,natural gas geochemical analysis,and fluid inclusion thermometric analysis,the geological conditions for hydrocarbon accumulation in the Formation were discussed.Meanwhile,coupling with the restoration of regional burial-thermal evolution history,the main factors controlling the hy-drocarbon accumulation were identified,and the reservoir evolution process was revealed.The results indicate that the Qixia Formation has good geological conditions for hydrocarbon accumulation.(i)the Qixia Formation dolomite reservoirs contain pores,vugs and fractures,with multi-stage superimposed thin layers vertically.(ii)Natural gas is mainly supplied by the high-quality source rocks of the Lower Cam-brian,partially by some source rocks of the Middle Permian,and by certain source rocks of the Silurian Longmaxi Formation(mainly in the southern part of the study area).(iii)The E-W strike-slip faults serve as favorable pathways for the vertical migration of natural gas,commu-nicating the source rocks and reservoir rocks to favor the gas accumulation.(iv)The gas reservoirs of the Qixia Formation are currently lith-ologic reservoirs under the tectonic setting.These reservoirs have undergone an evolution process from the Indosinian oil paleo-reservoir to the Yanshan gas paleo-reservoir and then to the present gas reservoir.In this process,the study area did not suffer remarkable adjustment and transformation,and reflects in-situ enrichment and efficient accumulation of gas.
The Middle Permian Maokou Formation has been one of the major gas-producing horizons in the Sichuan Basin for more than half a century.A new exploration upsurge targeting the dolomite reservoirs has been set off in recent years.However,the lack of system-atical study on the genetic mechanisms of various reservoirs limits progressive exploration.To clarify the characteristics and genesis of do-lomite reservoirs in the second member of Maokou Formation in the Longnvsi area of central Sichuan Basin,the genetic mechanisms of the reservoirs were investigated with the tectonism and using the data of outcrops,drilling,logging,and seismic interpretation.The results show that,(i)the dolomite reservoirs in the Maokou Formation are primarily composed of fine-to medium-crystalline dolomites formed by the dolomitization of beach grainstones;(ii)the reservoir spaces are dominantly made of hydrothermal impact-induced vugs and non-selective karst vugs,followed by structural fractures and karst fissures which play an important role in hydrothermal migration and reservoir seepage enhancement;(iii)the Hercynian Emei taphrogeny and the basement faulting controlled by the northward subduction of the Mian-lue Ocean induced the formation of the northern Sichuan platform basin in the Late Middle Permian,and controlled the distribution of beach grain-stones at the margin of the platform basin;(iv)affected by the high paleogeothermal gradient,remnant marine-originated fluids,and abun-dant magnesium ions from hydrothermal fluids migrating along faults,the beach grainstones were dolomitized extensively;(v)the Hercynian fault-fracture-hydrothermal activity and the Indosinian fault-fracture activity controlled the space formation of the dolomite reservoirs;and(vi)tectonism is the key controlling mechanism for the formation of dolomite reservoirs.The areas where faults are concentrated should be focused on for progressive exploration.
There developed varying dolomitized reservoirs in the Middle Permian Maokou Formation,central Sichuan Basin,in which significant gas discoveries have been made recently.Thus,the reservoir characterization,the enquiry of dolomitization mechanisms and the identification of main controls are very meaningful tasks for next exploration and deployment.Based on the macroscopic and microscopic observation on cores and outcrop samples,as well as some analysis on geochemical features,the characteristics,genesis and main controlling factors were investigated for the dolomite reservoirs.Results show that(i)these Maokou reservoirs in central basin are developed in the upper Maokou 2 Member,which are featured by stable lateral extension,and they are dominated by fine to medium crystalline residual sand-calstic dolomite,bioclastic dolomite,and dolomitic limestone.And there exist three reservoir types,for example,dolomite porous reservoirs,dolomite fractured-vuggy and limestone porous ones;(ii)a diagenetic sequence has been es-tablished according to petrological and mineral characteristics along with contact relationship.Eight diagenetic episodes were identi-fied,including contemporaneous exposure dissolution,contemporaneous dolomitization,shalllow-burial dolomitization,shallow-burial silicification,Dongwu dolomitization,Indosinian hydrothermal dissolution,Indosinian thermal silicification,and deep-burial dissolu-tion;and(iii)for the dolomite reservoirs of Maokou Formation in the study area,both extension and distribution are jointly controlled by platform-margin high-energy beach,early exposure dissolution and dolomitization.Both later hydrothermal dissolution and burial dissolution are conducive to reforming reservoirs.
