Study on the threshold value of organic enrichment of carbonate as gas source rocks

In this paper, calculations have been performed about gas quantity of generation, adsorption, dissolving in oil, dissolving in water, diffusion of unit area carbonate rocks at different geologic conditions in the Tarim basin. According to the material balance principle, the corresponding organic carbon content when gas started expelling from source rocks with separate phases has been worked out. We regard it as the theoretical threshold value (TOCmin) of gas source rocks under the same geologic condition. Based on the simulating calculation, a fact has been discovered that TOCmin decreases with the increasing source rocks thickness, decreases at the beginning and then increases with the increasing maturity and decreases with the better type of organic matter. TOCmin evaluation table of carbonate gas source rocks in the Tarim basin has been established. Investigations indicate that the TOCmin of carbonate gas source rocks varies greatly with the differences of geologic conditions, and gas source rocks cannot be evaluated with a unified TOC threshold value. And we also establish a preliminary evaluation table of TOC industrial threshold value, TOCgy, of carbonate gas source rocks in the Tarim basin.