Study On Reservoir Physical Parameters And Evolution Of The Chang-8 Member In The Londong Area Of Ordos Basin

Reservoir of the Longdong area, located in the southwestern of Ordos basin, had experienced complicated structural and sedimentary evolution history. The chang-8 member of Yanchang Formation Upper Triassic is one of the main exploration targets. The sandstones reservoir of Chang-8 has low porosity and low permeability whose features are of strong diagenesis, strong heterogeneity and complex pore structure.According to the geology characteristic and exploration difficulties, dynamic analysis clue was adopted in the paper. The eroded thickness of sediments was recovered under the background of structural and sedimentary evolution. Then the sedimentary and burial process of the basin after Mesozoic was rebuilt by Temis-3D basin modeling software of IFP. Based on research of reservoir characteristics and burial history, the process of diagenesis and the porosity evolution of reservoir were analyzed dynamically, and the characteristics of the reservoir in the key time of petroleum accumulation were acquired. The favorable aims of petroleum accumulation were forecasted finally.The eroded thicknesses of sediments, corresponding to four unconformities within Ceno-Mesozoic formations, were estimated mainly through the method of strata correlation. The burial history was reconstructed and results indicated that the strata in Longdong area had experienced four erosions in all after late Triassic, and the eroded thicknesses in Cretaceous is the largest. The sandstones reservoir of Chang-8 had experienced complicated deposition and burial process. The mean burial depth was 1500m before the erosion of the Late Jurassic. Then the strata was uplifted and eroded. The mean max burial depth was 2400m before the erosion of the Late Triassic.Based on other research about diagenesis and petroleum accumulation, the effects of compaction and cementation on reservoir physical properties were analyzed quantitatively, and then the physical properties of reservoir during the key time were recovered. Results indicated that the chang-8 sandstones reservoir had a good physical property with mean porosity was 17.35% which was close to the theoretical value. The chang-8 sandstones reservoir with good delivery capacity belonged to normal reservoir in Late Jurassic. Its characteristic of low porosity and low permeability and strong heterogeneity were formed by diagenesis in later burial process.Through comprehensive analysis, it is considered that the thickness of sand bodies and the connectivity in the key time were the critical factors of effective reservoir in the accumulation of oil. Through the parameters which were defined by sand bodies thickness multiplied Paleo-porosity, the distribution of sandstones reservoir which were efficient in petroleum accumulation period were predicted for next exploration.