| Author's Name: Hou Fanghao, Fang Shaoxian, HeJiang, Yang Xiyan, Fu Suotang,
Wu Zheng, Yao Jingli, Yan Ronghui |
| Ma51 to Ma54 Submembers that are at the uppermost part of Middle Ordovician Majiagou Formation are ones of the main nature gas pay horizons in Ordos Basin，According to the lithology and gas exploratory potential, each submember can be further divided into 2～4 layers which are 12 layers in total with total thickness of 70～80m.The best pay horizons
3 layers，and the next ones are Ma511, Ma514, Ma522, Ma541 layers. In lithological types of reservoirs, the predominant one is siltsized crystalline dolostone with anhydrite nodules or with anhydrite columnar crystalline and nodules,and the next is coarser silt-sized crystalline dolostone that contain intercrystal pores. Since experience for over 140 m.y. of sedimentary discontinuity form late Ordovician to Middle Carboniferous, the Ma51-Ma54 strata that situated at the topmost intensively suffered karstification of weathering palaeocrust during epidiagenetic exposure phase and formed a great amounts of solution pores，fissures，caves and channels.The secondary solution pores that formed during burial digenesis phase mostly stacked at the early paleaokastic pores and porous filling，or stacked at the coarser silt-sized crystalline dolostone with inter crystal pores so as to the stacked pore system developed. The good pore network that formed during Indo-Sinian and Yanshan movements formed in natural gas reservoir beds. By synthesizing the data of porosity, permeability, strata thickness, effective logging thickness, gas saturation and well flow from 399 wells, a criterion of reservoir evaluation is established. According to the criterion, the detailed reservoir evaluation does for each layer of Ma 51 to Ma 54 submembers and different reservoir grades of gas areas
are cycled and mapped. |
