Coupling relationship between tight sand reservoir formation mechanism and hydrocarbon charge in Shaximiao Formation of Tianfu gas field in the Sichuan Basin
Coupling relationship between tight sand reservoir formation mechanism and hydrocarbon charge in Shaximiao Formation of Tianfu gas field in the Sichuan Basin
Continuous breakthroughs have been made in the exploration and development of tight sand gas in the Middle Jurassic Shaximiao Formation of the Sichuan Basin over recent years,which shows a good prospect for natural gas exploration and development.However,the tightness of sandstone reservoir leads to a complex distribution of"sweet spots",especially the unclear understanding on the mechanism of reservoir densification and its coupling with hydrocarbon charge seriously restricts the scale and benefit development of tight sand gas reservoirs in the Shaximiao Formation.In this paper,taking the tight sand reservoir in the first member of Shahejie Formation(Sha-1 Member)of Tianfu gas field in the Sichuan Basin as an example,and based on various data of cores such as physical property test,thin section analysis,scanning electron microscope and geochemical test,the characteristics of tight sand reservoirs such material composition,reservoir property and diagenesis are systematically characterized,and the mechanism of reservoir densification is explored.Finally,combined with hydrocarbon accumulation stages,the coupling relationship between the reservoir densification process and the hydrocarbon charge is clarified.The results show that the reservoir of the Sha-1 Member is dominated by lithic arkose,with a small amount of feldspathic litharenite.The reservoir has a porosity of 0.82%-17.48%(avg.10.09%)and a permeability of 0.0 001-632.4 200 mD(avg.5.4 200 mD).The reservoir space is dominated by primary pores,with feldspathic dissolved pores and laumontite cement dissolved pores developed locally.The compaction of the Sha-1 Member is mainly weak to moderate,with chlorite incrust widely developed.The laumontite exhibits basal,porous and particle replacement cementation,the calcite exhibits basal and porous cementation,and the denudation is mainly composed of feldspar denudation and laumontite denudation.Three types of tight reservoir are developed in the Sha-1 Member:(1)tight reservoir formed under early strong cementation on the basis of compaction;(2)tight reservoir formed under late cementation on the basis of compaction;and(3)reservoir not tight nowadays,but tight at one time due to early compaction cementation,with its physical properties modified by later denudation.It is concluded that the reservoir of the Sha-1 Member has a single-phase hydrocarbon accumulation characteristic,occurring 88-68 Ma ago:effective hydrocarbon charge cannot happen in the first and third types of tight reservoirs,and can only happen in the second type of tight reservoir.The coupling relationship between the densification process of reservoir and the hydrocarbon charge in this area controls the relative enrichment of natural gas in the Jianyang block of the Tianfu gas field.
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