Reservoir forming conditions and favorable exploration zones in the Cambrian subsalt dolomite,Tarim Basin,NW China
The Cambrian subsalt dolomite has high quality source reservoir cap rock assemblage,which is an important substitute area for the further exploration and development in the Tarim Basin.However,due to the large buried depth and ancient strata,the exploration of Cambrian subsalt is faced with many basic geological problems,such as source rock,sedimentary reservoir and oil and gas preservation.On the basis of previous studies,through field outcrop,drilling analysis and regional seismic large section interpretation,this paper re-understands the Cambrian subsalt source rock,sedimentary reservoir,cap rock and oil and gas preservation conditions.Combined with the drilled well analysis,it points out the favorable exploration direction of Cambrian subsalt and optimizes the favorable exploration zone.The results show that:1)the source rock distribution of the Lower Cambrian Yuertus Formation is controlled by the Precambrian rift-depression system,mainly distributed in the northern depression and its surrounding areas,Southwest Tarim,with a total area of about 26.8×104 km2.2)There are two sets of reservoirs in the Lower Cambrian,namely,inner-middle gentle slope microbial mound and beach body of Xiaoerbulake Formation and reef back shoal of Wusongger Formation.The inner-middle gentle slope of Xiaoerbulake Formation is distributed along the periphery of Tanan ancient land,with an area of 4.5×104 km2.The reef back shoal of Wusongger Formation is developed in the inner side of Lunnan-Gucheng platform marginal zone,with an area of 5.3×103 km2.3)There are three facies belts in the Middle Cambrian gypsum salt rock caprock:gypsum salt lake,gypsum dolomite flat and argillaceous dolomite flat,which are distributed in a circular belt from the center of Bachu-Awati gypsum salt lake,with a total area of about 25×104 km2.4)Based on the analysis of drilled wells,it is proposed that the near source,high-quality reservoir cap rock combination and paleo-uplift background are the breakthrough direction of the Cambrian subsalt.Four class Ⅰ favorable areas,namely,Tazhong north slope,Gucheng-Xiaotangnan,Wensu paleo-uplift periphery and Lunnan area,and one class Ⅱ favorable area,namely,Maigaiti Slope,are selected,with a total area of 5.4×104 km2.The research results have a certain guiding significance for the next exploration of Cambrian subsalt dolomite in the Tarim Basin.
Tarim BasinCambrian subsaltXiaoerbulake FormationWusongger FormationMicrobial mound and beach bodyGypsum salt rockHydrocarbon accumulation
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