Tectono-Thermal Evolution, Hydrocarbon Filling and Accumulation Phases of the Hari Sag, in the Yingen-Ejinaqi Basin, Inner Mongolia,Northern China

This work restored the erosion thickness of the top surface of each Cretaceous formations penetrated by the typical well in the Hari sag, and simulated the subsidence burial history of this well with software BasinMod. It is firstly pointed out that the tectonic subsidence evolution of the Hari sag since the Cretaceous can be divided into four phases: initial subsidence phase, rapid subsidence phase,uplift and erosion phase, and stable slow subsidence phase. A detailed reconstruction of the tectonothermal evolution and hydrocarbon generation histories of typical well was undertaken using the EASY R% model, which is constrained by vitrinite reflectance(R) and homogenization temperatures of fluid inclusions. In the rapid subsidence phase, the peak period of hydrocarbon generation was reached at c.a.105.59 Ma with the increasing thermal evolution degree. A concomitant rapid increase in paleotemperatures occurred and reached a maximum geothermal gradient of about 43-45℃/km. The main hydrocarbon generation period ensued around 105.59-80.00 Ma and the greatest buried depth of the Hari sag was reached at c.a. 80.00 Ma, when the maximum paleo-temperature was over 180℃.Subsequently, the sag entered an uplift and erosion phase followed by a stable slow subsidence phase during which the temperature gradient, thermal evolution, and hydrocarbon generation decreased gradually. The hydrocarbon accumulation period was discussed based on homogenization temperatures of inclusions and it is believed that two periods of rapid hydrocarbon accumulation events occurred during the Cretaceous rapid subsidence phase. The first accumulation period observed in the Bayingebi Formation(Kb) occurred primarily around 105.59-103.50 Ma with temperatures of 125-150℃. The second accumulation period observed in the Suhongtu Formation(Ks) occurred primarily around84.00-80.00 Ma with temperatures of 120-130℃. The second is the major accumulation period, and the accumulation mainly occurred in the Late Cretaceous. The hydrocarbon accumulation process was comprehensively controlled by tectono-thermal evolution and hydrocarbon generation history. During the rapid subsidence phase, the paleo temperature and geothermal gradient increased rapidly and resulted in increasing thermal evolution extending into the peak period of hydrocarbon generation,which is the key reason for hydrocarbon filling and accumulation.

Relationship Between Paleogene Reservoir Densification and Hydrocarbon Accumulation in the Xihu Depression

Fluid inclusion analysis and testing were conducted to clarify the relationship between reservoir densification and hydrocarbon accumulation in the Paleogene Pinghu and Huagang formations in the Xihu Depression.The hydrocarbon accumulation stages of the reservoirs were studied in combination with the reconstruction results of burial and thermal evolution histories.Furthermore,the relationship between reservoir densification and accumulation charging was clarified in combination with the pore evolutionary history.In accordance with the time relation between reservoir densification and hydrocarbon charging,the reservoirs were classified into three types:pre-charging,syn-charging,and after-charging densification.Results indicated that large-scale hydrocarbon charging occurred in 11–0Myr.Reservoir densification was mainly caused by strong mechanical compaction and pore filling by well-developed siliceous and carbonate cements.Entering the middle diagenetic stage A1,the reservoir was under an acidic-diagenetic environment,resulting in the development of secondary dissolution pores.If large-scale hydrocarbon charging occurred during this period,then an after-charging densification reservoir,which is the most suitable type for hydrocarbon accumulation,might have developed.Entering the middle diagenetic stage A2,the reservoir was under an acidic-alkaline transitional diagenetic environment.During this stage,dissolution became weak,and compaction and cementation were enhanced,resulting in the continuous loss of pore space and reservoir densification.Entering the middle diagenetic period B,the reservoir was under an alkaline-diagenetic environment,and the reservoir had been largely densified.If large-scale hydrocarbon charging occurred during this period,a pre-charging densified reservoir,which is the worst reservoir type for hydrocarbon accumulation,might have developed.