Application of Fluid Inclusion Analysis for Buried Dissolution Predicting in the Tahe Oilfield of Tarim Basin, NW China

This study was undertaken to establish an evaluation method for buried dissolution of carbonate reservoir in the Tahe （塔河） Oilfield. Because of the difficulty in tracing the dissolution in geologic record, the precipitation is taken as a useful indicator to presume the dissolution reversed. The fluid inclusions data is a useful tool to identify the precipitation. On the basis of the principle that the temperature data of fluid inclusions captured in the calcite fillings of caves and cracks can reflect the fluid activity time, a total of 256 fluid inclusions samples in 12 drills were collected and analyzed to study their forming time in buried environment in the Tahe Oilfield, Tarim Basin, NW China. Results show that the temperature data of fluid inclusions captured by Ordovician cave and crack fillings accumulated in six intervals, which indicated six stages of fluid activity in the area of study. Meanwhile, it is clear that a different number of temperature intervals was captured in different wells, representing a different number of precipitation distributed in space. According to this conclusion, the buried precipitation trend in late Himalayan stage （the sixth stage） was supposed in space and its effectiveness has been approved by the chemical thermodynamics method. This method was also used in all of the other five stages of fluid activity and the comprehensive trend of the buried precipitation in space for Ordovician strata was evaluated. It shows that more temperature intervals were captured in the southwestern region （wells S76, S79, S65 and S75） and the eastern region （well S69） in the area of study, indicating intensive precipitation and weak dissolution in this area. This evaluation method for buried dissolution is a new attempt and may have important implications for further petroleum exploration in the Tarim Basin.

Origin and Distribution of Grain Dolostone Reservoirs in the Cambrian Longwangmiao Formation,Sichuan Basin,China

Dolostones in the Cambrian Longwangmiao Formation have become one of the most significant gas exploration domains in China. Over a trillion cubic meters of gas reservoirs have been discovered in the Gaoshiti-Moxi area; however, the origins and distribution of the dolostone reservoirs are not well understood. This work discussed the geology and geochemistry of the dolostone reservoirs in the Longwangmiao Formation to determine their origin and distribution. Two understandings are acquired： firstly, a carbonate ramp provided excellent conditions for grain beach deposition, while the presence of a hypersaline lake was favorable for the contemporaneous dolomitization of grain beach deposits. Petrographic and geochemical evidence further confirm that the Longwangmiao dolostone was formed during the contemporaneous stage. Secondly, the reservoir characteristics indicate that the grain beach sediments provide material basis for the development of the Longwangmiao dolostone reservoirs. Reservoir dissolution simulation experiments show that the porosity of the reservoirs was formed by dissolution during contemporaneous and burial stages. The dissolution pores formed during the contemporaneous stage were controlled by sequence interfaces. The large scale dissolution vugs formed during the burial stage subsequently spread along the pre-existing porosity and fracture zones. This study therefore identified that the development of grain dolostone reservoirs in a shallow water ramp under arid climatic conditions generally met the following conditions：（1） reefal beach deposits lay a foundation for reservoir development;（2） superficial conditions are an important determining factor for reservoir porosity; and（3） burial conditions provide environment for porosity preservation and modification.

Genetic mechanisms of secondary pore development zones of Es_4~x in the north zone of the Minfeng Sag in the Dongying Depression,East China

The genetic mechanisms of the secondary pore development zones in the lower part of the fourth member of the Shahejie Formation（Es_4／6x） were studied based on core observations,petrographic analysis,fluid inclusion analysis,and petrophysical measurements along with knowledge of the tectonic evolution history,organic matter thermal evolution,and hydrocarbon accumulation history.Two secondary pore development zones exist in Es_4~x,the depths of which range from 4200 to 4500 m and from 4700 to 4900 m,respectively.The reservoirs in these zones mainly consist of conglomerate in the middle fan braided channels of nearshore subaqueous fans,and the secondary pores in these reservoirs primarily originated from the dissolution of feldspars and carbonate cements.The reservoirs experienced ‘‘alkaline–acidic–alkaline–acidic–weak acidic＇＇,‘‘normal pressure–overpressure–normal pressure＇＇,and‘‘formation temperature increasing–decreasing–increasing＇＇ diagenetic environments.The diagenetic evolution sequences were ‘‘compaction/gypsum cementation/halite cementation/pyrite cementation/siderite cementation–feldspar dissolution/quartz overgrowth–carbonate cementation/quartz dissolution/feldspar overgrowth–carbonate dissolution/feldspar dissolution/quartz overgrowth–pyrite cementation and asphalt filling＇＇.Many secondary pores（fewer than the number of primary pores） were formed by feldspar dissolution during early acidic geochemical systems with organic acid when the burial depth of the reservoirs was relatively shallow.Subsequently,the pore spaces wereslightly changed because of protection from early hydrocarbon charging and fluid overpressure during deep burial.Finally,the present secondary pore development zones were formed when many primary pores were filled by asphalt and pyrite from oil cracking in deeply buried paleoreservoirs.