RESERVOIR QUALITY AND PETROPHYSICAL MODEL OF THE TARN DEEP-WATER SLOPE-APRON SYSTEM, NORTH SLOPE, ALASKA

The Tarn Field, North Slope, Alaska, lies southwest of the Kuparuk River Field and produces approximately 25,000 BOPD. The reservoir occurs as a stratigraphic trap along the base of slope within the Brooks Range foreland basin, and represents confined and unconfined slope-apron deposits of Cenomanian age. Sediment supplied to these systems ranges from mud-rich to mixed sediment and sand-rich sources. At maximum lowstand slope gullies fed two separate, but contemporaneous, slope apron systems.

The reservoir consists of very fine- to fine-grained, moderately- to well-sorted litharenites with an average composition of Q₁₀F₁₀L₈₀ and Ls₄₀Lv₂₀Lm₄₀. The sandstones consists largely of lithic grains of argillaceous sedimentary and metasedimentary detritus, and lesser amounts of epiclastic volcanic grains. They also contain a significant amount pyroclastic glass of intrabasinal origin that has altered to analcime. Analcime occurs as pseudomorphic replacement of glass shards, pumiceous fragments and vitrophyric grains and as spherulitic pore-filling cement. The pore system is largely primary with core porosities ranging from 4-28% and permeabilities form 0.1-50 md. Secondary intragranular porosity resulting from glass dissolution is a small but significant component of the pore system. Authigenic quartz rims on moldic grains are a by-product of dissolution and probably formed contemporaneous with analcime.

Reservoir distribution and characteristics are largely controlled by the depositional elements and sedimentary facies within the slope apron systems. The sand-rich systems have the best reservoir quality and connectivity. In mixed sediment systems, reservoir quality decreases from channel to lobe to levee deposits. Slope aprons confined in a sub-basin show greater reservoir connectivity than in unconfined settings.

The lithic nature of the reservoir presents unique challenges to log analysis. The variable mineralogies and physical characteristics of the lithic fraction are complicating factors, as is the low-density analcime. Grain densities vary from 2.52-2.78 g/cc and largely reflect the distribution of analcime and lithic grains. Petrologic data combined with routine core analyses were utilized to develop a log model for estimating porosity, permeability and saturation.