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Wednesday, January 24 2018

5:30 PM - 8 PM
Wednesday, Jan 24, 2018 5:30 p.m.
Intl. Dinner

Vendian Period Alluvial and Coastal Geomorphology Applied to Development of Verkhnechonskoye and Yaraktinskoye Fields, East Siberia

R. S. Tye 1, J. W. Hornbrook1, D. R. Lowe2, E. M. Prasse1, G. C. Robinson1, T. Royer1

1DeGolyer and MacNaughton, Dallas, Texas, U.S.A.

 2 Stanford University, Stanford, California, U.S.A.


Vendian-age (635-542 Ma) oil-bearing, Yarakta Horizon strata at the Verkhnechonskoye and Yaraktinskoye fields East Siberia, consist of conglomerate, sandstone, dolomitic sandstone, and mudstone fringing igneous-to-metasedimentary highlands of the East Siberia craton. Initial drainage networks formed within structurally defined valleys and early Yarakta Horizon deposition occurred in localized alluvial- to shallow-marine depositional systems. Base-level controlled depositional cycles aggraded the valleys; thus where interfluves were buried, broad alluvial and coastal plains formed.

Within one genetic cycle, facies associations grade basinward from alluvial (channel-bar, channel-fill, floodplain, crevasse-splay, and lacustrine) to shallow marine (sabkha, tidal-flat, estuarine-channel, and poorly developed beach/delta). 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. Thinly 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.

Three-to-seven bedsets of variable net-to-gross content comprise a parasequence. Depositional cycles varied locally, as ten and seven parasequences separated by decimeter to multi-meter thick mudstones are defined at Verchnechonskoye and Yaraktinskoye, respectively. Economic field development is hampered by the reservoirs’ channelized and onlapping stratigraphy, its effect on sandbody continuity and fluid levels, and diagenesis. 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 forms secondary pores. Owing to the abundance and variety of cements, grain composition, and their control on reservoir properties, unconventional petrophysical methods were required to accurately estimate clay content and porosity. Reservoir simulation, pressure-buildup, and interference tests suggest that these reservoirs can be profitably developed with high-angle and horizontal wells at 1,000 meter spacing.