Application of the Modified Method of Material Balance in Combination with Pyrolysis in Estimating the Hydrocarbon Generation Properties of the Bazhenov Formation, Western Siberia

Abstract

The paper analyzes methods currently used to evaluate the generation parameters of source rocks and suggests a new modified variant of the van Krevelen diagram and an original algorithm for evaluating the hydrocarbon generation properties of bitumen-bearing rocks of the Bazhenov Formation and its analogues. This is done with the application of an integrated approach that employs pyrolitic data on core samples and material-balance calculations. Data on the most probable values of the hydrogen index and the degree of transformations of the original organic matter into naphtides are suggested to use to formally subdivide the possible types of the organic matter into five groups. The paper presents processed analytical data on 340 samples from wells at 13 hydrocarbon fields and three areas in the Khanty–Mansi Autonomous Area and calculated original contents of organic matter in the rocks before naphtide generation was initiated in them, the original and current generation potential values, and the amounts of generated and migrated naphtides. The generation properties of the samples taken at minimum distances from one another (sampling sites spaced 1–3 cm apart) are demonstrated to broadly vary. The broad scatter of the generation properties of the samples cannot be explained only by that they contained organic matter of two types: terra- and aquagenetic. The broad variations in the generation characteristics of the rocks are thought to be likely explained by specifics of their microbial processing and variations in the redox parameters of sedimentation and diagenesis. The two probable reasons for the high S1/S2 ratios of the bitumen-bearing rocks are (1) the open porosity of the rocks and (2) active generation and sluggish migration of naphtides generated in these rocks. The very high contents of organic matter of the rocks (up to 40–50% and more) are explained by that they contain epigenetic thiobitumen, which were produced when these rocks were affected by high-enthalpy hydrothermal fluids. The minimum and maximum specific generation of naphtides in rocks of the Bazhenov Formation and its analogues are evaluated (per 1 km2, per 25 m rock thickness). The specific migration of naphtides generated in the Bazhenov Formation and its analogues are compared with the specific (per 1 km2) geological hydrocarbon reserves in the Khanty-Mansi Autonomous Area.