Research On Diagenesis And Diagenetic-hydrocarbon Filling Process Of Chang 8 Group In The Central Ordos Basin

The Chang 8 sandstone reservoir of the Upper Triassic in the Yanchang exploration area in the Ordos basin is with good oil & gas show and large explored reserves, exhibits a good exploration potential. But study on the mineral composition, physical properties and pore structure characteristics, diagenesis, hydrocarbon filling periods and time of Chang 8 sandstone is very weak. This paper uses a variety of analysis and testing methods to study the petrology and mineral components, pore geometry, types of diagenesis and diagenetic evolution sequence. The petrographic characteristics of fluid inclusions in the sandstones, hydrcarbon charging periods based on fluorescent features, homogenization temperature, chemical composition from the hydrocarbon-bearing inclusions, diagenetic-hydrocarbon filling process of the Chang 8 reservoir are also studied in this paper. The methods used in this study include identification and quantitative statistics of conventional thin sections and casting thin sections under microscope, fluorescence microscope analysis, X-ray diffraction of clay minerals, electron microscope scanning, laser Raman composition and homogenization temperature anaysis of fuild inclusions and so on. In order to make a correct understanding for the main impact factors led the sandstones to become the reservoirs with low porosity and low to very low permeability, probing fluid-rock interaction system and it’s temporal and spatial evolution under the diagenetic-hydrocarbon accumulation dynamic process, so as to get a further guidance on oil and gas exploration and development.The research results reveal that the sandstone types of Chang 8 in the study area are mainly lithic feldspar sandstone, are of low compositional maturity and intermidiate textural maturity as a whole, the interstitial materials are mainly carbonate and illite cements. The sandstone reservoir has become the reservoir with low porosity and low to very low permeability after mechanical compaction, cementation, dissolution and metasomatism, the average porosity and average permeability are 6.65% and 0.24mD, respectively. Compaction and cementation are the main factors which caused the porosity loss, resulting in 24.8% and 8.9% average porosity loss, respectively. Dissolution, causing 3.5% porosity (mainly the intergranular and feldspar intragranular dissolution pores) increase, improve the reservoir quality greatly.The Chang 8 sandstones in study area have generally entered the middle diagenetic A period. The sandstones experienced compaction, cementation, three stages of hydrocarbon charging, two phases of dissolution during this period, the diagenetic sequence is as follows:mechanical compactionâ†'micritic calciteâ†'clay pore lining (chlorite film â… )â†' hydrocarbon charging â… â†'quartz overgrowth â… â†'chlorite film â…¡â†' illite/smectiteâ†'dissolution â… â†'hydrocarbon charging â…¡â†'quartz overgrowth â…¡â†'sparry calcite â… â†'kaoliniteâ†'intergranular of illite, chlorite cementationâ†'laumontiteâ†'structural fracture forming â†'dissolution â…¡â†'hydrocarbon charging â…¢-*sparry calcite â…¡.The research result of the diagenetic-hydrocarbon filling evolution process of Chang 8 sandstone, based on reservoir diagenesis, fluid inclusion characteristics and hydrocarbon filling, the chronologic sequence of diagenetic-hydrocarbon filling evolution, analysis of stratigraphic burial-thermal history, shows that the Chang 8 sandstones has experienced three periods of hydrocarbon filling-diagenetic evolution process:the first phase of hydrocarbon filling, with limited proportion, occurred in the early diagenetic A-B period of the Middle Jurassic (about 169-161Ma, with the buried depth of 1200-1400m), the hydrocarbons mainly filled in the primary pores, in the fluid inclusions corresponding to this filling period didn’t found traces of hydrocarcons. The second hydrocarbon filling period occurred in the early diagenetic B period from the late Jurassic to the early Cretaceous (148-135Ma, with the buried depth 1500～1900m), the hydrocarbons are mainly distributed in the grain boundaries with small amount of in the dissolution pores, the peak temperature of the corresponding fluid inclusions is 70～90℃. The third phase of hydrocarbon filling occurred in the middle diagenetic A-B period duing the late Early Cretaceous (125.2～105.7Ma, with the buried depth 2300～3100m). A large amount of the hydrocarbons distributed in variety dissolution pores and cracks, it’s the main hydrocarbon accumulation period. After that the late sparry calcite cementation made the sandctones to become reservoirs with low porosity and low to very low permeability.