Using Resistivity Log Invasion Profiles, Multiple Porosity Logs, Core and Test Data to Determine Lithology, Porosity and Permeability in Pre-Cambrian Sandstones of Eastern Siberia (Russian)
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Ediacaran-age (635–542 Ma) oil-bearing strata in the Yarakta Horizon at the Verkhnechonskoye and Yaraktinskoye fields, East Siberia, consist of conglomerate, sandstone, dolomitic sandstone, and mudstone overlying and onlapping igneous to metasedimentary highlands of the East Siberia craton. Initial drainage networks formed within structurally defined valleys, and early deposition occurred in localized alluvial to shallow-marine depositional systems. Base-level-controlled depositional cycles aggraded the valleys; thus, as valleys aggraded, they buried interfluves and coalesced forming broad alluvial and coastal plains. Three to seven bedsets of variable net-to-gross content constitute a genetic cycle. Depositional cycles varied locally, as nine and eight cycles separated by decimeter- to multi-meter-thick mudstones are defined at Verknechonskoye and Yaraktinskoye, respectively. Within one genetic cycle, facies associations grade basinward from alluvial (channel-bar, channel-fill, floodplain, playa, and crevasse-splay) to shallow marine (sabkha, tidal-flat, estuarine-channel, and poorly developed shoreface). Coarse-grained lithofacies are typically arranged in decimeter- to meter-scale bedsets with sharp to scoured bases. Bedsets commonly, but not always, show an upward decrease in grain size, bed thickness, and scale of sedimentary structure. Typically, medium-grained sandstones exhibit low-angle cross bedding and are gradationally overlain by fine-grained sandstones exhibiting scour-and-fill, cuspate-ripple lamination, climbing-ripple lamination, and parallel lamination. Clay clasts and small pebbles are accessories. Interbedded mudstones, siltstones, and sandstones show ripple cross bedding, wavy to lenticular bedding, abundant soft-sediment deformation (e.g., shear, fluid-escape, slump features), and slickensides. Thin-bedded sandstones are micaceous and contain granule-size mud chips. Some mudstones exhibit crinkled to parallel laminae indicative of algal growth. Sandstone fills mudcracks. Interbedded green and black mudstones, plus pyrite and siderite cements, indicate alternating redox conditions. Alluvial facies have patchy quartz, anhydrite, and carbonate cements. Marine-influenced facies show early and well-developed quartz cement as well as abundant halite. Gypsum and halite dissolution formed secondary pores. Calculated estimates of fluvial-channel dimensions and sinuosities indicate that despite the lack of vegetation, fluvial channels in the Yarakta Horizon were shallow and relatively narrow, moderately sinuous, and exhibited varying degrees of mud-prone overbank deposition. Recognition and correlation of flooding surfaces and channel diastems bounding genetically related strata identified multiple stratigraphic compartments in each field. Porosity loss at chronostratigraphic boundaries accounts for complex water, oil, and gas contacts. Economic field development is hampered by locally varying reservoir quality and sandstone continuity caused by its channelized and onlapping stratigraphy and diagenesis. Reservoir simulation of varying geostatistical models demonstrate that differing porosity-distribution methods had little effect on estimates of in-place hydrocarbon volumes. Model differences in porosity and permeability distribution and lithofacies connectivity show large variations in recovery factor and productivity/injectivity.
Ediacaran-age (635–542 Ma) oil-bearing strata in the Yarakta Horizon at the Verkhnechonskoye and Yaraktinskoye fields, East Siberia, consist of conglomerate, sandstone, dolomitic sandstone, and mudstone overlying and onlapping igneous to metasedimentary highlands of the East Siberia craton. Initial drainage networks formed within structurally defined valleys, and early deposition occurred in localized alluvial to shallow-marine depositional systems. Base-level-controlled depositional cycles aggraded the valleys; thus, as valleys aggraded, they buried interfluves and coalesced forming broad alluvial and coastal plains. Three to seven bedsets of variable net-to-gross content constitute a genetic cycle. Depositional cycles varied locally, as nine and eight cycles separated by decimeter- to multi-meter-thick mudstones are defined at Verknechonskoye and Yaraktinskoye, respectively. Within one genetic cycle, facies associations grade basinward from alluvial (channel-bar, channel-fill, floodplain, playa, and crevasse-splay) to shallow marine (sabkha, tidal-flat, estuarine-channel, and poorly developed shoreface). Coarse-grained lithofacies are typically arranged in decimeter- to meter-scale bedsets with sharp to scoured bases. Bedsets commonly, but not always, show an upward decrease in grain size, bed thickness, and scale of sedimentary structure. Typically, medium-grained sandstones exhibit low-angle cross bedding and are gradationally overlain by fine-grained sandstones exhibiting scour-and-fill, cuspate-ripple lamination, climbing-ripple lamination, and parallel lamination. Clay clasts and small pebbles are accessories. Interbedded mudstones, siltstones, and sandstones show ripple cross bedding, wavy to lenticular bedding, abundant soft-sediment deformation (e.g., shear, fluid-escape, slump features), and slickensides. Thin-bedded sandstones are micaceous and contain granule-size mud chips. Some mudstones exhibit crinkled to parallel laminae indicative of algal growth. Sandstone fills mudcracks. Interbedded green and black mudstones, plus pyrite and siderite cements, indicate alternating redox conditions. Alluvial facies have patchy quartz, anhydrite, and carbonate cements. Marine-influenced facies show early and well-developed quartz cement as well as abundant halite. Gypsum and halite dissolution formed secondary pores. Calculated estimates of fluvial-channel dimensions and sinuosities indicate that despite the lack of vegetation, fluvial channels in the Yarakta Horizon were shallow and relatively narrow, moderately sinuous, and exhibited varying degrees of mud-prone overbank deposition. Recognition and correlation of flooding surfaces and channel diastems bounding genetically related strata identified multiple stratigraphic compartments in each field. Porosity loss at chronostratigraphic boundaries accounts for complex water, oil, and gas contacts. Economic field development is hampered by locally varying reservoir quality and sandstone continuity caused by its channelized and onlapping stratigraphy and diagenesis. Reservoir simulation of varying geostatistical models demonstrate that differing porosity-distribution methods had little effect on estimates of in-place hydrocarbon volumes. Model differences in porosity and permeability distribution and lithofacies connectivity show large variations in recovery factor and productivity/injectivity.
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.
The Verkhnechonskoye oil and gas field in Eastern Siberia produces from a highly complex Precambrian, lower Vendian sandstone. In this reservoir there has been significant post-depositional alteration due to diagenesis, secondary porosity is common and salt, anhydrite and carbonate cementation of the porosity frequently occurs. In short, the reservoir provides substantial petrophysical challenges. In the development of a petrophysical model three specific concerns were noted. Firstly, the effects of the salt-filled pore spaces complicate the determination of porosity and therefore permeability. Secondly, it is difficult to resolve permeability predictions from logs, core and formation testers. Finally, using conventional petrophysics, even with a comprehensive suite of logs and extensive core data, it is difficult to clearly delineate gas-filled porosity from oil-filled porosity and the determination of the gas-oil contact was therefore uncertain. This paper will discuss how petrophysical data from traditional triple-combo measurements was integrated with NMR measurements, core analysis data and formation tester data including permeabilities from pressure transient analysis (PTA) to produce a coherent and robust petrophysical model for permeability prediction.
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