Study On Mechanism Of Deep Gas Accumulation In Bonan-Gubei Area

On the basis of the documentary research on the reservoir-forming characteristics of the typical deep gas reservoirs all around the world, applying theories on Gas reservoir-forming, and the combination of various radical informations, lots of techniques, such as basin modelling, high accuracy fluid inclusion testing and analysis, reservoir bitumen analysis, are fully used to analyse the reservoir-forming characteristics of Bonan-Gubei area, Jiyang Depression, including the reservoir's formation period, hydrocarbon migration pathway, the distinguishment of the gas genetic model and the dynamic procedure of reservoir forming.Here in this paper, the formation time and stage of the deep gas reservoir are ascertained by a set of analysis techniques, including the hydrocarbon generation history, fluid inclusion homogenization temperature testing, light hydrocarbon characteristic and structure movement analysis. Subsequently, two periods of different original gas chargements consisting of coal-formed gas chargements in middle stage of Ng and oil-formed gas chargements in early stage of Nm in Gubei buried hill belt are established. At the same time, there are two accumulation periods which include from late stage of Es1 to late stage of Ed and from early stage of Ng to nowadays in oil-associated gas reservoirs in Bonan sub-sag, the accumulation of late stage of Nm plays important part, and the accumulative phase is dominated by structure apparently, the fomation period of gas reservoir at downstructure location is earlier than that of gas pool at upstructure location.From the geological setting, after studying the pattern and mechanism of migration factors which include sand bodies, faults, fractures and so on, according to the alteration characteristic of the geochemical datas of natural gas, degree of mineralization of formation water, reservoir bitumen, the fault effeciency and the biomarkers in the migration pathways, it can be easily to obtain the recognition that gas migration system categories with different genetic mechanisms and in different areas are different: cracked gas in Bonan sub-sag and coal-formed gas in GuBei buried hill migrate along sand bodies mostly, and the migration direction of the latter is dependent on structural attitude. Cracked gas in the block belt travels along the fault chiefly. In addition, the migration effectiveness in different sections of a fault is different.The origin identification of natural gas demonstrates that the region of interest has multiple sources and different origin types of gas: coal-cracked gas from Carboniferous-Permian source rocks migrates into the trap in Gubei buried hill, kerogen-cracked gas mainly appears in Boshen 4 fault blocks; and whereas oil-cracked gas dominates in deep-zone of Bonan sub-sag and Boshen 6 buried hill zone. Associated with high accuracy fluid inclusion testing and the analysis of GOI, it may be easy to get the conclusion that the palo-gas/water contact is above the present gas/water contact in well Gbg-1, which may indicate a combination of different types of gas accumulation and an increasement in the gas migration quantity in later stage,along with favourable preservation.On the foundation of deep natral gas reservoir-forming mechanism, two types of reservoir formation pattern are established respectively: one source, muti-types and polyphase reservoir-forming pattern in Bonan area and muti-sources, muti-types and polyphase reservoir -forming pattern in GuBei area. The gas making up the deep gas reservoir in Bonan sub-sag almost comes from the same source rocks in Es4s, which consists of kerogen-cracked and oil-cracked gas, and with twice chargement periods including from late stage of Es1 to late stage of Ed and from early stage of Ng to nowadays. At the same time, the deep gas reservoir in Gubei aera is composed of the gas from Es4s source rocks in Bonan sub-sag and Carboniferous-Permian gas source rocks in Gubei buried hill zone, and it has the character of different types of gas migrating into the trap at different periods, while the chargement dimention reaches maxinum in late stage of Nm.