Characteristics and Signifificance of Acid-Soluble Organic Matter in Marine Carbonate Source Rocks

Since the 1950s, great breakthroughs have been made in the field of marine carbonate oil and gas exploration, and many countries have discovered large reservoirs in carbonate formations. Foreign scholars have carried out systematic research on the formation and evolution of marine hydrocarbon source rocks, mainly evaluating the hydrocarbon source rocks by conventional methods or means such as organic matter type, maturity, organic matter abundance, etc., supplemented with hydrocarbon simulation results to study the mechanism of hydrocarbon generation and discharge of the hydrocarbon source rocks and evaluate the hydrocarbon potential, which has effectively guided the practice of oil and gas exploration in the marine stratigraphic system. However, the marine facies generally experienced multiple phases of uplift and denudation and subsidence processes, the effectiveness of highly evolved carbonate source rocks has been controversial, posing new challenges to some of the classical theories and traditional understanding that have long guided oil and gas exploration. At present, the index for evaluating the organic matter abundance of carbonate hydrocarbon source rocks is mainly TOC.According to the evaluation criteria of different scholars at home and abroad on the lower limit of hydrocarbon production of TOCin carbonate rocks, marine carbonate rocks in most regions of China cannot reach the standard of hydrocarbon source rocks. Previous studies have found that when adding acid to remove carbonate minerals in TOC pretreatment, a large amount of organic matter will be lost with the acid solution, resulting in low TOC results. In order to address the problem of low TOC values caused by the loss of acid solution in conventional TOC testing, we established a new method for testing the organic matter abundance in carbonate hydrocarbon source rocks by using montmorillonite thickening method, which effectively avoids the loss of acid-soluble organic matter, an important hydrocarbon forming material, in the process of TOC testing, and has achieved good results. The results showed that most of the samples measured by the new method were higher than the conventional TOC results, and there was no correlation between the acid-soluble organic matter content and TOC. Due to the loss of acid-soluble organic matter, the evaluation of hydrocarbon source rocks in highly evolved carbonate rocks should also take into account the species and number of hydrocarbon forming organisms, the organic carbon isotope composition and the degree of negative drift of inorganic carbon isotopes in order to effectively identify and evaluate the hydrocarbon source rocks of highly over-mature carbonate rocks. Therefore, the study of the forms of acid-soluble organic matter, formation conditions and evaluation methods is helpful for us to understand the organic matter composition of carbonate source rocks and their hydrocarbon evolution more comprehensively, and these understandings are of good significance in guiding the oil and gas exploration of carbonate rock formations.