Appraisal of a Precambrian Oil and Gas Field: Verkhnechonskoye Field, East Siberia, Russian Federation

Abstract

To evaluate uncertainty in property distribution and reservoir connectivity, resource size, and development plans for Verkhnechonskoye, a large Precambrian oil field, three geologic models were built employing:Sequential Indicator Simulation (SIS) and Sequential Gaussian Simulation (SGS);Object Modeling; andSeismic-Attribute Modeling. Reservoir models were based on seismic and wireline-log, core, and well-test data from over 100 wells. The reservoir consists of sandstone, mudstone, and rare conglomerate deposited on a broad alluvial plain fringing an igneous and metamorphic continental island. Coarse-grained sediments were deposited in channels that prograded and aggraded the alluvial plain that terminated downdip into a lacustrine or epicontinental sea. In places, lagoons and poorly developed beaches fringed the alluvial plain. Relative base level created upward-coarsening and upward-fining successions that onlap the basement. Shale beds create flow barriers while, porosity and permeability are primarily controlled by evaporate, quartz, and calcium carbonate cements. Buildup and interference tests were designed using estimated average reservoir properties. Test results discriminated among the three geologic models for use in field-development planning. Partial field-simulation models were upscaled from each geologic model and used to match the buildup and interference tests. The SIS model surpassed the Object and Seismic models in matching well performance. This better match of well performance from the SIS model occurred because sandstones are better connected and exhibit more homogeneous rock properties as compared to the Object or Seismic models. It was concluded that the SIS method of facies and property distribution is preferred for future modeling in the field. Introduction The Precambrian Verkhnechonskoye field is located approximately 600 kilometers north of Irkutsk, Eastern Siberia, in the Russian Federation (Fig. 1). To assist in evaluating uncertainty in property distribution and reservoir connectivity, resource size, and development planning, 3-D geocellular models of terrigeneous-clastic reservoirs in the Verkhnechonskoye field, were built using data from wireline logs, 2D and 3D seismic, cores and porosity/permeability analyses, and well tests. While an extensive exploration and appraisal program had established the existence of a significant hydrocarbon accumulation, the distribution of reservoir properties and reservoir connectivity were poorly understood. Both complex reservoir characteristics and uncertainties in existing data sets contributed to the lack of understanding of reservoir properties. Three geocellular models were constructed: one based on statistical distribution of wireline-log data (SIS model); the second, an object model using relationships derived from core descriptions (Object model); and a third based on seismic attributes (Seismic model). Specifically, the models constitute three geologic descriptions of the reservoir to test:the uncertainty in reservoir oil-in-place estimates;reservoir connectivity; andfield-development plans. Volumetric variation was shown to be small whereas, reservoir connectivity and property distribution varied significantly depending upon the modeling approach employed. Stratigraphic and sedimentologic interpretations relied heavily on cores from six wells. Wireline logs (100+ wells; Fig. 1) calibrated to cores provided lithology, porosity, permeability, and water saturation values distributed in the models. Interval property maps, structure maps, fault planes, and seismic facies were generated from 2D and 3D seismic data.