A NEW METHOD FOR CARBON ISOTOPE PREDICTION IN SUB-SALT CARBONATE SOURCE ROCKS

Geochemical parameters, as a part of studying hydrocarbon generation potential in sub-salt carbonate reservoirs, are very crucial for later oil-gas field evaluation. At present, studies have shown that carbon isotopes reflect the mutual conversion of organic and inorganic carbon, proving that some organisms have been converted into hydrocarbons. Therefore, we believe that factors such as the degree of carbon isotope negative drift, especially the part with large negative drift, can be considered to evaluate the hydrocarbon generation potential of sub-salt carbonate source rocks.

Traditionally, carbon isotopes were obtained through experimental analysis of cored samples. But the experimental data obtained by this method is limited and discrete. Well logging data has the advantage of high resolution, abundance, and continuity. Therefore, we propose to establish a logging interpretation model to predict the carbon isotope in sub-salt carbonate source rocks.

We continuously sampled and tested carbon isotopes from Majiagou Formation from a well in the central and eastern Ordos Basin, and found that the range of value range has a good correlation with conventional logging, especially the radioactivity curve. Subsequently, it was found that the value range of carbon isotopes changed greatly in the open platform dominated by dolomite, while the change in the restricted platform dominated by limestone was small, because the gypsum developed in the open platform and thermochemical sulfate reduction (TSR) occurred, resulting in negative drift of carbon isotopes. Therefore, we used conventional logging data, fully considered the changes of sedimentary environment, and chose local polynomial regression to predict carbon isotopes.

And it is also proposed a carbon isotope value of <-2‰ should be considered indicative of effective source rocks when the lower limit of total organic carbon content is 0.2%, with the potential for increased thickness of the source rocks. The results of the study lead to better understand the hydrocarbon generation history of ultra-deep carbonate source rocks, which is of great significance for future to evaluate the effective source rocks of high and mature carbonate rocks.