Study On Reservoir Characteristics And Main Controlling Factors For M₅⁵ Sub-member Of The Ordovician Majiagou Formation In Sulige Area

The M₅⁵ sub-member of Ordovician Majiagou Formation is the latest successive horizon of natural gas exploration in Lower Paleozoic of Sulige area. However, the research of M₅⁵ sub-member reservoir develops laggardly when compared with other members which have been made a greater exploration of the various strata of Majiagou Formation, so it is necessary to carry out a comprehensive and systematic study on the reservoir's characteristics and main controlling factors of the M₅⁵ sub-member.According to the theoretical basis of sedimentary, petroleum geology, carbonate rock reservoir geology, geophysics theory, with core observation, thin section, SEM, mercury intrusion and other laboratory data analysis as a means as well as geophysics and geochemistry, papers were made to have a systematic study on strata, rock, deposition, diagenesis, tectonic evolution of the M₅⁵ sub-member of Ordovician Majiagou Formation in Sulige area, analyzed the main controlling factors which influence the formation and development of the reservoir, finally predicted the distribution of favorable reservoir. The specific understandings are listed as follows.Depositional environment is a preferential and decisive factor for the formation of favorable reservoir. M₅⁵ sub-member of Majiagou formation is identified to be carbonate platform facies through drilling and logging data, sedimentary background, previous studies analysis, mainly developing restricted platform subfacies and open platform subfacies, and dolostone flat, limestone flat, point bars, dolomitic limestone flat are the principal microfacies. Furthermore, dolostone flat, evaporative gymsum dolostone flat contain various dolostone, which are the favorable sedimentary facies for reservoir development.Diagenesis is a critical guarantee to form favorable space in reservoir. When reffered to the reservoir development of M₅⁵ sub-member affected by diagenesis, dolomitization is one of the most essential types. The M₅⁵ sub-member was taken as a case to systematically analyze forming conditions and genesis of dolomite through cast sections, SEM, CL, XRD, with isotope of C, O, Sr, REE and Macro and Trace Metals analysis as geochemical approches, arriving at a conclusion that contemporaneous dolomitization and burial dolomitization are the two genetic types. Micrite dolostone, siltcrystal-micrite dolostone mainly develop in the process of contemporaneous dolomitization while siltcrystal dolostone forms in the process of burial dolomitization. The favorable reservoir mainly distributes in the dolostone reservoir of the research area. Within various diagenesi in study area, filling and cementation, metasomatism, compaction, pressure-solution take destructive effects to the reservoir development, nevertheless, dolomitization and dissolution play a favorable part to the formation of the reservoir porosity.Tectonic movement is a favorable supplementary factor for reservoir development, on the one hand, it provides geomorphological conditions for karstification which is advantageous to reservoir development, on the other hand, cracks which developed in tectonic movement could improve physical properties of the reservoir.The main lithology of reservoir contain siltcrystal-micrite dolostone, siltcrystal dolostone; Intercrystalline pore, dissolved pore, anhydrite moldic pore and fracture are the main reservoir space. The reservoir physical properties of M₅⁵2 small layer is superior than that of M₅⁵1 small layer. After the analysis work of reservoir classification and evaluation of M₅⁵ sub-member, it is evident to get the conclusion that the reservoir can be divided into three types??, ?, ??. And type I is good reservoir, type is relatively good reservoir, type? is bad reservoir.Based on the comprehensive research, the favorable reservoirs are mainly located in the northern and western regions of the study area, which mostly showed sheet distribution characteristics.