Kenneth P. Helmold¹, Wayne J. Campaign¹, William R. Morris¹, Douglas S. Hastings¹, Steven R. Moothart¹

(1) Phillips Alaska, Inc, Anchorage, AK

ABSTRACT: Petrology, Petrophysical Model, and Controls on Reservoir Quality in the Tarn Deep-Water Slope-Apron System, North Slope, Alaska

The Tarn Field encompasses a Cenomanian slope-apron turbidite system within the Brooks Range foreland basin. Early lowstand deposits are muddy unconfined slope aprons containing slump blocks and lobes of debris/slurry flows. At maximum lowstand, slope gullies fed two contemporaneous sand-rich slope-apron systems that comprise the reservoir.

The reservoir sandstones consist of very fine- to fine-grained, moderately- to well-sorted litharenites with an average composition of Q11F16L73 and Ls40Lv33Lm27. They consist largely of argillaceous sedimentary and metasedimentary detritus and contain a significant amount pyroclastic glass altered to analcime. The analcime occurs as pseudomorphic replacement of glass shards, pumice and vitrophyric grains and as spherulitic pore-filling cement. EPMA data show the analcime is a pure Na-Al hydrous silicate with trace amounts of calcium, potassium and iron. The pore system is largely primary with core porosities ranging from 4-27% and permeabilities from <0.01-289 md. Secondary intragranular porosity, resulting from the dissolution of glass, is a small but significant component of the pore system. Authigenic quartz rims on moldic grains are a by-product of dissolution and formed contemporaneously with analcime. Grain densities vary from 2.52-2.78 g/cc and largely reflect the distribution of analcime and lithic content. Petrologic and petrophysical data were utilized to develop a log model for estimating porosity, saturation and permeability.

Reservoir distribution and properties are largely controlled by depositional elements, grain size and sedimentary facies within the slope apron systems. The sand-rich systems have the best connectivity, with reservoir quality progressively decreases from channel -> lobe -> crevasse splay -> levee deposits.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado