We clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma5^5-Ma5^10 sub-members of the Ordovician Majiagou Formation in the Ji...We clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma5^5-Ma5^10 sub-members of the Ordovician Majiagou Formation in the Jingxi area in the Ordos Basin： （1） Syngenetic microbial dolomitization is characterized by formation of dolomite with a mainly micrite structure and horse tooth-shape dolomite cements. （2） Seepage reflux dolomitization during the penecontemporaneous period superposed adjustment functions such as recrystallization and stabilization in the middle-deep burial stage, forming dolomites mainly consisting of micro crystal and powder crystal structure. （3） Powder dolomite, fine dolomite, and medium-coarse crystalline dolomite formed in pores and fractures in the middle-deep burial stage. The secondary concussive transgression-regression under a regressive background is an important condition for the occurrence of many stages of dolomitization in the study area. The basin was an occlusive epicontinental sea environment in the Ma5 member of the Ordovician Majiagou Formation sedimentary period. In the sediments, sulfate content was high, which is conducive to the preservation of microbial activity and microbial dolomitization. Micritic dolomite formed by microbial dolomitization provides good migration pathways for seepage reflux dolomitization. Affected by evaporation seawater with increased Mg/Ca ratio, seepage reflux dolomitization was widely developed and formed large-scale dolomite, and underwater uplifts and slopes are favorable areas for dolomite. In the middle-deep burial stage, dolomitizing fluid in the stratum recrystallized or stabilized the previous dolomite and formed a small amount of euhedral dolomite in the pores and fractures.展开更多
The Upper Paleozoic in the north part of Tianhuan depression in the Ordos Basin,NW China has lower hydrocarbon generation intensity and complex gas-water relationship,the main factors controlling the formation of tigh...The Upper Paleozoic in the north part of Tianhuan depression in the Ordos Basin,NW China has lower hydrocarbon generation intensity and complex gas-water relationship,the main factors controlling the formation of tight sandstone gas and the distribution of tight sandstone gas in the low hydrocarbon generation intensity area are studied.Through two-dimensional physical simulation experiment of hydrocarbon accumulation,analysis of reservoir micro-pore-throat hydrocarbon system and dissection of typical gas reservoirs,the evaluation models of gas injection pressure,reservoir physical property,and gas generation threshold were established to determine the features of tight gas reservoirs in low hydrocarbon intensity area:(1)at the burial depth of less than 3 000 m,the hydrocarbon generation intensity ofis high enough to maintain effective charging;(2)tight sandstone in large scale occurrence is conducive to accumulation of tight gas;(3)differences in reservoir physical property control the distribution of gas pool,for the channel sandstone reservoirs,ones with better physical properties generally concentrate in the middle of sandstone zone and local structural highs;ones with poor physical properties have low gas content generally.Based on the dissection of the gas reservoir in the north Tianhuan depression,the formation of tight gas reservoirs in low hydrocarbon generating intensity area are characterized by"long term continuous charging under hydrocarbon generation pressure,gas accumulation in large scale tight sandstone,pool control by difference in reservoir physical property,and local sweet spot",and the tight gas pools are distributed in discontinuous"sheets"on the plane.This understanding has been proved by expanding exploration of tight sandstone gas in the north Tianhuan depression.展开更多
基金the Fundamental Research Funds for the Central Universities(14CX02116A)the Foundation of State Key Laboratory of Petroleum Resources and Prospecting,China University of Petroleum,Beijing(No.PRP/open-1604)
文摘We clarified three stages of dolomitization and secondary changes by studying the petrology and geochemistry characteristics of dolomite from the Ma5^5-Ma5^10 sub-members of the Ordovician Majiagou Formation in the Jingxi area in the Ordos Basin： （1） Syngenetic microbial dolomitization is characterized by formation of dolomite with a mainly micrite structure and horse tooth-shape dolomite cements. （2） Seepage reflux dolomitization during the penecontemporaneous period superposed adjustment functions such as recrystallization and stabilization in the middle-deep burial stage, forming dolomites mainly consisting of micro crystal and powder crystal structure. （3） Powder dolomite, fine dolomite, and medium-coarse crystalline dolomite formed in pores and fractures in the middle-deep burial stage. The secondary concussive transgression-regression under a regressive background is an important condition for the occurrence of many stages of dolomitization in the study area. The basin was an occlusive epicontinental sea environment in the Ma5 member of the Ordovician Majiagou Formation sedimentary period. In the sediments, sulfate content was high, which is conducive to the preservation of microbial activity and microbial dolomitization. Micritic dolomite formed by microbial dolomitization provides good migration pathways for seepage reflux dolomitization. Affected by evaporation seawater with increased Mg/Ca ratio, seepage reflux dolomitization was widely developed and formed large-scale dolomite, and underwater uplifts and slopes are favorable areas for dolomite. In the middle-deep burial stage, dolomitizing fluid in the stratum recrystallized or stabilized the previous dolomite and formed a small amount of euhedral dolomite in the pores and fractures.
基金Supported by the China National Science and Technology Major Project(2016ZX05047)
文摘The Upper Paleozoic in the north part of Tianhuan depression in the Ordos Basin,NW China has lower hydrocarbon generation intensity and complex gas-water relationship,the main factors controlling the formation of tight sandstone gas and the distribution of tight sandstone gas in the low hydrocarbon generation intensity area are studied.Through two-dimensional physical simulation experiment of hydrocarbon accumulation,analysis of reservoir micro-pore-throat hydrocarbon system and dissection of typical gas reservoirs,the evaluation models of gas injection pressure,reservoir physical property,and gas generation threshold were established to determine the features of tight gas reservoirs in low hydrocarbon intensity area:(1)at the burial depth of less than 3 000 m,the hydrocarbon generation intensity ofis high enough to maintain effective charging;(2)tight sandstone in large scale occurrence is conducive to accumulation of tight gas;(3)differences in reservoir physical property control the distribution of gas pool,for the channel sandstone reservoirs,ones with better physical properties generally concentrate in the middle of sandstone zone and local structural highs;ones with poor physical properties have low gas content generally.Based on the dissection of the gas reservoir in the north Tianhuan depression,the formation of tight gas reservoirs in low hydrocarbon generating intensity area are characterized by"long term continuous charging under hydrocarbon generation pressure,gas accumulation in large scale tight sandstone,pool control by difference in reservoir physical property,and local sweet spot",and the tight gas pools are distributed in discontinuous"sheets"on the plane.This understanding has been proved by expanding exploration of tight sandstone gas in the north Tianhuan depression.
