| As one of the essential hydrocarbon production sections of many large basins in the world,the research on hydrocarbon accumulations of carbonate-evaporite series is always one of the hotspots of discussion.In this paper,taking the Tarim evaporite basin in Cambrian,a typical craton basin in China,as an example,the geotectonic background,the paleoclimate conditions,and the paleo-sea level were summarized based on previous data.The petrological characteristics,the sedimentary palaeogeographic evolution,the paleoenvironment,the formation mechanism of carbonate-evaporite series,and the distribution of evaporites were discussed by means of mineral identification and element/isotope geochemistry.On these bases,the characteristics of reservoirs and source rocks were studied,the "salt-reservoir relationship" and "salt-source relationship" were discussed,the source-reservoir combination model was established,and the evaluation and prediction of favorable hydrocarbon accumulation areas were completed.The evaporite series of the Middle-Lower Cambrian in the Tarim Basin mainly developed evaporites,carbonates,and claystones,and developed sedimentary facies such as carbonate ramp,carbonate-evaporite platform,platform margin,slope,and basin.Carbonate platform includes open platform,restricted platform,and evaporitic platform.The carbonate-evaporite series in the craton basin is the product of the coupling of tectonic,material source,and paleoclimate.Among them,the fluctuation of periodic rise-stagnation of paleo-sea level mainly controls the sedimentary sequence of carbonate-evaporite series in the craton basin.The carbonate deposits were mainly developed in the transgression stage of paleo-sea-level rise;the interbedded deposits of carbonates and evaporites were mainly developed in the early high-level period of intermittent alternation of paleo-sea-level rise and stagnation;the evaporite deposits were mainly developed at the end of the high-level period or the retrogression stage.The spatial distribution of evaporites is limited by such factors as the fluctuation of paleo-sea level,the barrier action of microbial reefs and shoals on the wide ocean,and the paleo-topography.Referred to the results of modern seawater evaporation and salt precipitation,combined with the paleo-seawater environmental index and the lithology sequence of carbonate-evaporite series,the salinity range of carbonate-evaporite series in craton basin is divided into several salinity intervals,such as medium-high salinity interval dominated by carbonates such as limestone and dolostone,and high/ultra-high salinity interval dominated by a large amount of evaporites,such as gypsum and halite.Based on the petrological study of carbonate-evaporite series,the reservoir rock types,reservoir space,diagenetic evolution,and reservoir porosity distribution of carbonate-evaporite series and their relationship with evaporites were analyzed.It is clear that the reservoir formation of carbonate-evaporite series in the craton basin is controlled by sedimentary facies,diagenesis,structure,and evaporites.It was found that the dolomites interbedded with or adjacent to the evaporites were affected by the secondary dissolution of each diagenetic stage,and they were easy to form the molds of evaporitic minerals or various types of solution pores and cracks.The adjacent fault systems can also be filled and blocked or re-dissolved due to evaporitic minerals.Combined with the division of paleo-seawater salinity,it was considered that the reservoir facies of carbonate-evaporite series mainly includes the shoals of the restricted platform formed in the medium-high salinity interval and the dolomitic plats of the evaporitic platform in the high salinity interval.Under the restriction of the sedimentary facies of the carbonate-evaporite series,the Middle-Lower Cambrian source rocks in Tarim Basin were further divided into platform source rocks and non-platform source rocks.The non-platform source rocks are mainly claystones formed in the ramp,slope,and deep-water basin from low salinity to medium-low salinity,while platform source rocks are mainly argillaceous carbonates and gypsum claystones interbedded with evaporites in the restricted platform or evaporitic platform with medium-high salinity.Through the analysis of the evaluation parameters and major/trace elements of the platform source rocks,it is found that the platform source rocks of the Middle-Lower Cambrian carbonate-evaporite series in the Tarim Basin mainly show type Ⅲ kerogen with a poor content of organic matter.Platform source rocks are comprehensively controlled by paleo-salinity,paleo-redox environment,and paleo-seawater depth.Among them,the frequent fluctuation of sea level in the early high-level stage makes the platform seawater fluctuate back and forth between low salinity and medium-high salinity,which is more conducive to the enrichment and preservation of organic matter.Finally,based on the above research results and the comparative analysis of carbonate-evaporite series in typical craton basins,the source-reservoir assemblage of carbonate-evaporite series of Middle-Lower Cambrian in the Tarim Basin was analyzed.Taking the sedimentary facies,the thickness of source rock,the thickness of gypsum rock and gypsum mixture,the distribution of fault zones,and the source-reservoir assemblage as the main reference factors,the division criteria of three types of favorable areas for hydrocarbon accumulations are established.The favorable areas of hydrocarbon accumulations of the Xiaoerbrak Fm.,the Wusongger Fm.,and the Awatag Fm.were predicted.Among them,the favorable areas of Type Ⅰ are distributed in the salt pan of the Awatag Fm.and the Wusongge Fm.,and most areas of the Bachu uplift and the Central-Tarim uplift of the Xiaoerbrak Fm.,the favorable areas of type Ⅱ are mainly distributed in the periphery of type Ⅰ and the North-Tarim uplift,and the favorable areas of type Ⅲ are mainly distributed in the marginal areas of platform source rocks. |
