ANALYSIS OF K, U, TH, AND B DISTRIBUTION IN THE UPPER JURASSIC SEDIMENTS OF THE CENTRAL PART OF THE KRASNOLENINSK ARCH (WESTERN SIBERIA) WITH THE AIM OF STRATIFICATION, CORRELATION, AND SEPARATION OF PAY SANDS

Abstract

The discovery of hydrocarbon reservoirs in Upper Jurassic sediments in the central part of the Krasnoleninsk arch required stratification of these deposits and separation of its oil reservoirs; correlation of their sections penetrated by wells; and, finally, prediction of their areal distribution over the field or separate pools. Poor knowledge of these sediments hampered the solution to these problems until recently. The stratification of these sediments proposed in this paper and the separation of various lithological types including reservoir rocks is based on an analysis of the distribution of the naturally radioactive element (NRE) and total radioactivity of Upper Jurassic rocks. The reservoir rocks are termed potentially productive layers (PPL). The eight zones with specific lithologic and mineralogic compositions previously distinguished in Upper Jurassic sediments and basic lithological types of rocks in these zones (including PPL) were used for the analysis. The investigations demonstrated that well gamma-ray spectroscopy helps the stratification of Upper Jurassic sediments; the separation of PPL; and, when integrated with neutron logging, the assessment of PPL volume capacity, which is necessary for the estimation of hydrocarbon reserves. The application of the results of paleogeographic reconstruction of Upper Jurassic sedimentation enabled us to propose a prediction approach for the areal distribution of PPL of various compositions. The reconstruction was based on an analysis of variations of the Th/U ratio and B contents in rocks composing different zones of these sediments.