Causes And Reservoir Of Dolomite In The Cretaceous Langshan Formation In Geji From Tibet, China

The Cuoqin Basin is the second marine basin in Tibet , the tectonic position of which is between Bangonghu-Nujiang suture zone and Gangdise magmatic arc . With the deep study of petroleum geology of the basin , lack of good reservoir is an important issue of oil and gas evaluation in local area. Based on the petrological characteristics, geochemistry characteristics, reservoir property analysis of Langshan Formation , we achieve the following understanding:1)According to the relative content of calcite and dolomite , the dolomite of Cretaceous Langshan Formation which belongs to the Cuoqin Basin Geji section can be divided into three categories : dolostone , calcareous dolostone and limestone bearing dolostone , according to the grain size of the dolomite , dolostone can be divieded into medium-fine–grained dolostone and fine-grained dolostone.2)The microscopic characteristics and cathodoluminescent lights of dolomite indicate that the dolomites in this study area all formed by replacement of calcite , but not original .Though the results of stable carbon and oxygen isotope , Strontium isotope , cathodoluminescent light , X-ray diffraction and trace elements , combing with salinity index Z and forming temperature which are calculated from data of stable carbon and oxygen isotope captured in dolostone imply that they are the products of burial compaction-squeeze and dolomitization metasomatism .3)Diagenetic phenomena in this area are more complex and are always changing . The main diagenesis observed through the microscope are pressure solution compaction , filling , dolomitization , recrystallization , limonitization , corrosion and fracturing related structure. Among them, the dissolution, dolomitization and the major structural cracks play a constructive role, while compaction, such as the role of filling the reservoir making the damaging effects.4)Research on Cretaceous Langshan Formation dolomite reservoir properties are good : 11 samples of dolomite reservoir porosity rangs from 2.6-25.1% , with the average porosity of 10.72% ; Permeability varies from a value of 0.12 -2360×10-3μm2, the average penetration rate is 19.51×10-3μm2 (excluding maximum 2360×10-3μm2) , overall , they belong to the low porosity and medium permeability reservoir . Casting thin sections combined with microscope and mercury curve analysis , dolomite reservoir pore space are abundant , and the principal is fractured– porous , with the mainly pore structure of pores in the throat - coarse pore large throat , the overall evaluation isⅠ-Ⅱclass better - good reservoir .