Study On The Strata And Sedimentary Facies Of The Fourth Member Of The Sinian Lamp In The Gaoshiti-Moxi Area

Recently, as a huge exploration breakthrough has been made in Deng 4 member of Sinian System in Gaoshiti-Moxi Area, Deng 4 Member is considered to have favorable exploration prospect. According to current researches, the development of high quality reservoirs of Deng 4 Member in Gaoshiti-Moxi Area is controlled by sedimentary facies and karstification. Grain shoal and mud mounds are widespread in Deng 4 Member and karstification of Tongwan period impact on the reservoir quality significantly. However, there are few detailed studies about stratigraphy distribution, conservation, and spatial distribution of sedimentary facies, which have affected high quality reservoirs prediction. Therefore, it's important to study stratigraphy and sedimentary facies of Deng 4 Member for prediction high quality reservoirs in Gaoshiti-Moxi Area. Based on geological setting and the analysis of about 30 drilled and log wells,11 cored wells and more than 200 thin sections, according to the latest stratigraphic classification scheme from PetroChina Southwest Oil and Gas Field Company, guided by the sedimentary cycle and combined with the characteristics of lithology, logging and fossil, this study divides the Deng 4 Member's stratigraphy. Integration of the theories of sedimentology, logging geology and reservoir geology, this paper also researches the types of lithology, sedimentary facies distribution and favorable sedimentary belts of Deng 4 Member. Due to the karstification of Tongwan Period, the western part of Deng 4 Member was under heavy erosion in Gaoshiti-Moxi Area, even leading to sequence missing, but the retained thickness of Deng 4 member is 200m to 340m, which is characterized by distribution tendency of thickest in the western region, thinner in the mid-eastern region and thicker in the eastern region, which indicated that Deng 4 Member increased thick near denudation line. The depositional setting of Deng 4 Member in Gaoshiti-Moxi Area is carbonate platform with amount of algal dolomites, grain dolomites crystalline dolomites deposited, containing tidal-fiat subfacies, restricted lagoon subfacies, mud mound subfacies, grain shoal subfacies, mound-shoal complex subfacies and more than 10 micro facies. Mud mound subfacies, grain shoal subfacies mound-shoal complex subfacies are dominant facies in Gaoshiti-Moxi Area. Vertically, the thickness of upper Deng 4 Sub-member is thicker than low Deng 4 Sub-member. Laterally, the western Deng 4 Member are better than eastern parts in strata continuity. Horizontally, in the low Deng 4 Sub-member period, because of the ancient landform in Moxi Area was relatively low, mound-shoal complex facies mainly distributed in Gaoshiti Area. In the upper Deng 4 Sub-member period, mound-shoal complex facies developed well in western Moxi Area and formed continuous banded distribution to Moxi Area and southern Gaoshiti Area. Based on the reservoir properties analysis, reservoirs' porosity in mound-shoal complex facies is best, reservoirs'porosity in mud mound subfacies and grain shoal subfacies takes the second place, and that in tidal-flat subfacies and lagoon subfacies is the worst. As a result, mound-shoal complex facies is considered as most favorable sedimentary facies belt.