Characteristics And Genesis Of Paleogene Gypsum-rocks In The Central Kuqa Depression

The deposition of the Kumglemu Group in the center of the Kuqa Depression within the Tarim Basin is featured in thick gypsum-salt bed,which is considered as a high-quality caprock for oil and gas reservoirs.Due to tectonic compression,the plastic deformation of the gypsum-salt bed caused huge changes of horizontal thickness,which brought a series of problems for geological exploration and drilling engineering.Through the comprehensive combination of drilling,logging and seismic data,the petrological characteristics,logging response characteristics,geochemical characteristics and horizontal distribution of the Kumglemu Group gypsum-salt rocks were analyzed by observation of cores,thin sections,scanning electron microscopy and whole-rock X-ray diffraction.On this basis,the genesis of gypsum-salt rocks in the research area was further understood.As results shown,（1）The Kumglemu Group in the central part of Kuqa Depression is mainly composed of gypsum-salt rocks,carbonates rocks and terrestrial clastic rocks.According to the mineral compositions,the main types of gypsum-salt rocks in this paper include two kinds:gypsum rocks and salt rocks.Among them,the gypsum rocks are divided into seven types,which are gypsum rocks,gypsum rocks with dolomite,dolomitic gypsum rocks,gypsum rocks with lime,calcite gypsum rocks,gypsum rocks with mud,and argillaceous gypsum rocks.Salt rocks are divided into four types:pure stone salt rocks,salt rocks with mud,argillaceous salt rocks and gypsum-salt rocks.（2）Different lithologies display different logging response characteristics.The gypsum rocks are featured in low natural gamma values,medium to high resistivity and significant diameter shrinkage.The salt rocks are featured in extremely low gamma value,high resistivity with large amplitude differences and common diameter expansion.（3）The Kumglemu Group can be divided into five lithologic sections from the bottom to the top:lower mudstone section,gypsum-salt rock section,middle mudstone rock section,salt rock section and upper mudstone rock section,which have different characteristics in different areas.The lower mudstone section is mudstone depositional assemblage in the Awa-Bozi area and Kela-Keshen area,and silty mudstone in the Dabei area.As for the gypsum-salt section,it is an association of salt rock-gypsum rock-dolomite/limestone-gypsum rock in the Awa-Bozi area and Kela-Keshen area,which is 2～3 sets of dolomite/limestone depositional assemblage in the Dabei area.In the middle mudstone rock section,a stable thickness of mudstone deposition exhibits in the research area expect in the Dabei 1#well area.The salt rock section is made up of laminated gypsum-salt rocks in the Awa-Bozi area and Kela-Keshen area,but pure salt rocks in the Dabei area.The upper mudstone rock section is dominated by mudstone deposition,but there are more sandstones in Kela-Keshen area and Awa-Bozi area.The northern part of Dabei area also contains a small amount of sandstones,while in the rest part,a thin layer of gypsum is laminated.（4）According to the textural characteristics of mudstone and its mineral composition,combined with the geochemical indicators of gypsum-salt rocks and mudstones,such asδ³⁴S and ⁸⁷Sr/⁸⁶Sr,Mg/Ca,Fe and Al,and（Fe+Al）/（Ca+Mg）,it can be concluded that the gypsum-salt rocks of the research area are formed by evaporation and deposition of the salt lake under the dry climate conditions,and that the salt minerals derived from the multiple stages of transgressions in the southwestern direction.In terms of ancient landform,vertical lithological sequence and geochemical characteristics,it is clear that the gypsum-salt rocks in the research area were formed under the influence of transgression,and a series of salt-lakes were isolated by multiple paleo-uplifts.The overall deposition evolutionary process is that the salt lakes gradually shrank and dried up after several times of rapid transgressions,and then desalinated by the continuous injection of terrestrial freshwater.Under the effect of such frequent supply of seawater and repeated“shrinking”of salt lakes,salt rocks of great thickness were eventually deposited in the research area.