Quantitative evaluation of the sealing ability of gypsum-salt rock caprocks

The reserves of hydrocarbon reservoirs with gypsum-salt rock as cap rock account for 55% of the total hydrocarbon reserves in the world. The influence of cap rock sealing ability is an important problem to be solved in hydrocarbon accumulation analysis. Gypsum-salt rock belongs to evaporite, which is often associated with mudstone, limestone and dolomite, resulting in extremely complex lithologic composition of gypsum-salt rock caprock. At present, the analysis of sealing capacity of gypsum-salt rock caprock lacks the distinction between gypsum rock and salt rock, and the previous comprehensive evaluation of sealing capacity of caprock mainly focuses on the macro and micro characteristics of caprock. As one of the influencing factors of sealing capacity - mechanical properties, they are ignored in the evaluation system of sealing capacity of caprock.

Based on this, the classification standard of gypsum-salt rock caprock type is established, and the gypsum-salt rock caprock is divided into "three categories and five sub categories". Comprehensively considering the macro and micro characteristics and mechanical properties of the caprock, the lithology and rock combination type, lithology zoning, cumulative thickness of dominant lithology, caprock thickness, maximum thickness of thick single layer, ground coverage ratio, internal friction coefficient, tensile strength and peak strength are selected as the evaluation factors. The analytic hierarchy process (AHP) is introduced to determine the weight of each factor, formulate the evaluation standard, and establish the evaluation system of gypsum-salt rock caprock based on AHP to realize the quantitative evaluation of the sealing capacity of gypsum-salt rock caprock.

The evaluation method is applied to the Cambrian gypsum salt caprock in Tarim Basin, China. The results show that the C value in Bachu area of Tarim Basin is between 2.6-3.9, with an average value of 3.3, and the caprock quality is good; The C value in Tazhong area is 1.8-22, with an average value of 2.0, and the sealing capacity of caprock is generally general; The C value of Tabei uplift is less than 2, and it is in the range of poor caprock as a whole. On the whole, the evaluation value of sealing capacity of gypsum-salt rock caprock in Tabei—Tazhong—Bachu area gradually increases and the sealing capacity gradually becomes better. Combining the evaluation results with the vertical distribution of hydrocarbon, the corresponding vertical display type of hydrocarbon is mainly the distribution type under salt rock, and the corresponding hydrocarbon distribution position of poor caprock is above salt. The evaluation results are highly correlated with the vertical distribution of hydrocarbon, It shows that the evaluation system is of great significance for accurately evaluating the sealing hydrocarbon ability of gypsum-salt rock caprock and guiding hydrocarbon exploration.

Keywords: gypsum-salt caprock; lithological combination; sealing ability; quantitative evaluation; Tarim Basin