Study Of Hydrocarbon Preservation Conditions In Nanpanjiang Depression

Based on previous studies, the structural evolution in Nanpanjiang Depression wasstudied using seismic data, log data, geochemical data and others. Consequently, hydrocarbonpreservation conditions and their main control factors were discussed, the favorablepreservation units were divided, and the hydrocarbon exploration directions were defined.Cap rock sealing ability was analyzed in consideration of macroscopic and microscopicconditions, including displacement pressure, depositional environment, lithologiccharacteristics, diagenetic stages, etc. The middle and upper Tertiary strata, upper Permianstrata, lower Carboniferous strata and middle to upper Devonian strata are good cap rocks inNanpanjiang depression. Cpa rocks of middle to upper Tertiary strata are the best, upperDevonian to lower Carboniferous strata are better, and middle to upper Devonian strata arethe worst. The depositional history of Nanpanjiang depression since Devonian was dividedinto 3 hydrogeologic cycles and 6 steps, and the underground water is mainly NaHCO3 typefrom Devonian to Tertiary strata, which indicates normal sealing conditions.The vertical hydrogeologic characteristics show that the cap rocks were in free exchangezone and retarded exchange zone. In terms of the fluid dynamics analysis in fault zone usinghydrochemical methods, the opening degree of main fault zone , the influences of fault andfracture activities on hydrocarbon preservation conditions were studied. The higher theopening degree of large fault zone is, the poorer the hydrocarbon preservation condition is.Because of the tectonic uplift movements, the cap rocks suffered strong erosions and thehydrocarbon dissipated. During the periods of Indochina and Yanshan-Xishan movements,Nanpanjiang depression uplifted and was eroded. The eroded strata in Indochina movementperiod were relative thin, and the destructions of reservoirs were weaker. During Yanshan toXishan movement periods, the depression uplifted again and was eroded heavily, and thedestruction of reservoirs was intensive, with the eroded strata inside sags were thinner.Considering the comprehensive influences of cap rocks, fault activities, structural uplifts, andmagmatic activities on preservation conditions, preservation units were divided, and thefavorable exploration zones were predicted.