An improved time-depth dual porosity evolution model and a new parameter for tight sandstone reservoir quality evaluation

Paleoporosity recovery is important for an in-depth understanding of the densification process of tight sandstones and the evaluation of reservoir quality. In this study, an improved time-depth dual porosity model is proposed for the quantitative characterization of porosity evolution. This model incorporates the dual effects of time and burial depth on porosity evolution and determines the reduced or increased porosity caused by different diagenesis such as compaction, cementation, and dissolution. This model innovates by conducting a quantitative analysis of the evolution of porosity reduced by cementation. Taking the first member of the Shihezi Formation (P1x1) of the Upper Paleozoic in the Hangjinqi area of the Ordos Basin as an example, the initial porosity of P1x1 sandstones in Well J118 was determined to be 40.4%, the decreased porosities by compaction and by cementation were 30.9% and 8.5%, respectively, and the increased porosity by dissolution was 7.3%. Excess porosity, which is the difference between reservoir porosity and critical porosity in the main hydrocarbon accumulation period, is proposed as a new and effective parameter for evaluating tight reservoir quality. Based on the excess porosity, tight reservoirs can be classified into three levels: (1) Class I reservoirs with excess porosity greater than 4.0% have good quality and daily gas production in excess of 10.0 x 103 m3/d, (2) Class II reservoirs with excess porosity of 2.0%-4.0% have medium quality and daily gas production of 1.0 x 103-10.0 x 103 m3/d; and (3) Class III reservoirs with excess porosity below 2.0% has poor quality and daily gas production of less than 1.0 x 103 m3/d.