--> Diagenesis and Reservior Quality in the Upper Triassic Xujiahe Formation, Yuanba Area, Northeastern Sichuan Basin, China

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Diagenesis and Reservior Quality in the Upper Triassic Xujiahe Formation, Yuanba Area, Northeastern Sichuan Basin, China

Abstract

The Upper Triassic Xujiahe Formation in Sichuan Basin is one of the typical tight gas reservoirs in China with hundreds of billions demonstrated reserves since explored from the 1950s. Except Central and Western Sichuan Depression the exploration is still in low degree, presently. Xujiahe Formation in Yuanba area with lower degree of exploration has met several prolific wells in recent years. Therefore, investigating relatively high quality reservoirs has a practical value for further exploration. Based on well cores, thin sections, cathodeluminescence (CL), scanning electron microscope (SEM), well logs (Acoustictime, Density, Dual Lateral logs) and petrophysical data, diagenetic facies and associated log responses were undertaken to unravel the controls on reservoir quality and the distribution of relatively high quality reservoirs. The Xujiahe sandstones are litharenites containing calcarenaceous sandstones, and quartzarenites. Important diagenetic processes that affected the sandstones include compaction, dissolution of feldspar and/or alteration to clay, precipitation of carbonate and quartz cements, precipitation of illite and chlorite, and alteration of detrital clays (mixed-layer illite-smectite). Some conclusions can be obtained as follows: 1. Four types of diagenetic facies can be classified by the cement content: siliceous cementation facies (SCF, siliceous cements more than 80% of total cements), calcitic cementation facies (CCF, calcitic cements more than 80% of total cements), clay mineral cementation facies (CMCF, clay mineral range from 50 to 80% of total cements) and transition facies (TF, contained variable cements with none predominating). 2. Reservoirs of CMCF and TF are relatively high quality with mean porosity of 5.21% and 4.1%, and mean permeability of 0.15 mD and 0.14 mD, respectively. Reservoirs of SCF and CCF are much more density with mean porosity of 2.53% and 1.83%, and mean permeability of 0.05 mD and 0.02 mD, respectively. 3 CCF are recognized by high density values (DEN), abnormally high dual lateral values (RS and RD) and low acoustictime values (AC). SCF also has high DEN and low AC, but has no obvious of RS and RD response. CMCF and TF are recognized by their low DEN and high AC with no obvious RS and RD response.