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Model to Predict Tight-gas Sands Porosity of West Depression in Liaohe Basin, China


The Eocene–Oligocene Es3 member of Shahejie formation in our study area is a feldspathic, debris-rich and tight gas sandstone reservoir (porosity 2.4–10.4%, permeability 0.09 −0.38 md), a consequence of depositional attributes (grain composition, size and sorting) acted upon by diagenesis (significant mechanical and chemical compaction, precipitation of carbonate cements and authigenic clays, and deep-burial cementation by quartz) throughout the time. Considering these impacts, a comprehensive porosity prediction model is developed based on the correlation between sedimentary facies and diagenesis. The result could also provide for the exploration of other tight gas reservoir. Based on the analyses of sedimentology and petrography data of 80 cored wells, the study investigated the distribution and influences of porosity, established a model by the following steps: (1) Diagenesis index (ID) was established integrating with temperature, Ro, quartz overgrowth, I/S and depth and its relationship with sandstone porosity was determined to build the exponential porosity model. (2) Sedimentary facies index (IF), which was higher in the advantageous sedimentary facies, was calculated by the distribution of porosities in different sedimentary facie during each diagenetic stage. (3) The ultimate porosity model based on the relationship between the exponential porosity model and sedimentary facies (IF) was developed since the porosity was the cumulative effects of sedimentary facies and diagenesis. This improved model can also restructure the evolving history of the reservoir aside from predicting present porosity. The improved model was applied to 16 wells from slope to sag throughout the field. A comparison of predicted and measured porosities showed and 1.84% average absolute error with the pore-filling of 16%. This indicates the model may be used elsewhere to predict porosities. The results show that: porosity decreased sharply by compaction during the early diagenetic stage; secondary porosity developed in the middle diagenetic stage A1-A2; the reservoir became tighter with the continuous compaction alongside with quartz and carbonate cement. The reservoir in the middle diagenetic stage A1 can be described as conventional and stage A2 unconventional.