Conversion of the Oil and Gas Generation Potential of the Deep Source Formations of the West Siberian Basin: An Example of Well Tyumen SG-6

Abstract—

A model of the thermal evolution of the lithosphere of the West Siberian Basin in the area of Superdeep Well SG-6, which was drilled through the Koltogor–Urengoi graben, is used to numerically estimate the generation of various hydrocarbon (HC) fractions by the Triassic and Jurassic source rocks. The thermal model assumes the emplacement of a sill into the subsurface layers of the basement in the Early Jurassic and hydrothermal activity in the Late Pliocene–Lower Pleistocene, which noticeably impacted the conversion history of the HC potential of the Triassic and Jurassic source rocks. For the Triassic Pur formation, the emplacement of the sill into the subsurface layers of the basement in the Lower Jurassic drastically intensified the conversion of the HC potential to 84% and the degradation of more than 97% of the generated light oil mass. Calculations show that the heavy oil generated by the rocks of the Pur, Togur, and lower horizons of the Tyumen formations has degraded almost completely as a result of secondary cracking, while heavy oil predominates among the generated HC fractions in the upper horizons of the Tyumen Formation and in the rocks of the Bazhenovo Formation. The light oil remaining nowadays in the matrix of the source rocks has completely degraded in the Triassic rocks but makes up much of the HC products in rocks at the bottom of the Togur Formation and the top of the Tyumen Formation. This oil is the dominant in the upper horizons of the Togur Formation and in the bottom part of the Tyumen Formation. According to calculations, gas hydrocarbons account for a significant proportion of HC products in the Togur and Tyumen formations, and they dominate in the Middle Triassic Pur Formation. At the relatively low initial potential of HC generation and a low content of organic matter in the rocks of the Pur, Togur, and Tyumen formations, no primary expulsion threshold of liquid HC has been reached, and the generated liquid HC probably did not leave the rock matrix, while the gaseous HC likely migrated. The threshold of primary expulsion of liquid HC for the Bazhenov rocks was calculated to be reached at about 65 My.

APPENDIX KINETIC SPECTRA OF KEROGEN CRACKING

As has been mentioned, the kinetic spectrum of kerogen cracking of any given source formation was based on the four-component spectra of three standard kerogen types, which were compiled from the TemisSuite 2008 database: marine kerogen of type II with HI = 611 mg HC/g TOC (Menil-2002), kerogen of type II with a poorer initial potential HI = 377 mg HC/g TOC, and terrestrial kerogen of type III with HI = 160 mg HC/g TOC (Mahakam III). The kinetic spectra for the cracking of kerogens of the types within the framework of a four-fraction model have been worked out at the French Institute of Petroleum (Institut Français du Pétrole, IFP) and are applied in the widely used MATOII program package for basin modeling. These spectra are presented in Tables 2 and 3. Note that the parameters of the secondary cracking reactions of heavy and light oil are the same for kerogen of type II and III. The four-fraction kinetic spectrum of type-II kerogen cracking with a poor initial potential HI = 377 mg HC/g TOC was analogous to the spectrum in Table 2, except that the initial potentials of the primary cracking reactions were normalized to the total potential of 377 mg HC/g TOC.

Table 2. Kinetic parameters of the reactions of four-fraction cracking of marine kerogen of type II with an initial potential of 611 mg HC/g TOC (French Institute of Petroleum, data from ThemisSuite 2008, Menil-2002)

Table 3.   Kinetic parameters of four-fraction cracking reactions of continental kerogen of type III with an initial potential of 160 mg HC/g TOC (French Institute of Petroleum, data from ThemisSuite 2008; Mahakam III)