Heavy Oils from the West Sak Field, North Slope, Alaska: Geochemical Oil-Property Prediction and Columnar Gradients

B. Huizinga¹, M. Werner², Z. Chen¹, A. Holba¹, E. Michael¹, S. Redman², and Bret Chambers^3
1ConocoPhillips, Houston, TX
²ConocoPhillips, Anchorage, AK
³BP Alaska, Anchorage, AK

The West Sak heavy-oil field (12-23^o API), containing 8 billion barrels of original oil-in-place in Upper Cretaceous reservoirs, was mostly delineated in the 1980’s during the development of the deeper Lower Cretaceous-reservoired Kuparuk conventional oil field. It was common practice at that time to co-mingle drill-stem tests from independent adjacent oil reservoirs and to supplement this fluid sampling with conventional coring of the reservoirs, using differing drilling fluids and core-sample preservation for each well. We have recently developed a biomarker method, based on aromatic compounds, that successfully predicts API gravity and viscosity from these historic cores, regardless of their condition, whether or not their light-ends have been lost due to evaporation or the oils contaminated by drilling-fluid additives. This geochemical method has been benchmarked, and blind tested, by comparing aromatic results from frozen sidewall cores and MDT fluid samples (PVT analyses), and gives confidence that sidewall core samples can be used alone to assess oil properties in specific zones not covered by MDT sampling. Besides effectively using older delineation core data, the aromatic biomarker method reduces costs by 10-15 times and increases sampling density by 5-10 times, when compared to using MDT fluid sampling alone to appraise oil properties in recent wells.

Our API gravity prediction makes use of a multi-variate equation optimized from aromatic compounds measured from a library of oils with known properties. When applied to reservoir extracts, we found in-situ oils to behave somewhat differently for oils recovered from wells on primary or water-flood production, so the API calibration has been tailored to the current production mechanism. West Sak oil-quality is strongly dependent on two natural processes: (1) oil-biodegradation intensity and (2) secondary recharge of light hydrocarbons, probably vertically leaked from the underlying Kuparuk field. Development of a multi-variate API equation is essential to more accurate oil-property prediction, and is clearly superior to older single-parameter approaches using an oil-biodegradation ratio alone. Our predicted API gravities are then converted to live-oil viscosities via a field-specific transform utilizing API gravity, GOR, and reservoir temperature.

As the reservoirs become shallower (lower temperature), oil-biodegradation intensity generally increases and light-end recharge tends to decrease, resulting in observed differences between reservoirs (oil quality from best to worst: WS-A > WS-B > WS-D > Ugnu). However, each individual oil column shows distinct oil-property gradients, which are fundamental for high-grading areas with the best oil quality. A variety of gradient types occur in different parts of the field, including: (1) non-linear API gradients with best oil-quality at trap crest (high-graded area) but progressively deteriorating API down-column toward the OWC (very common gradient type, where oil fills the sand and pushes a limited OWC-area down the flank of the structure); (2) better quality oil possibly isolated in the structural crest by down-flank faulting (high-graded area; oil biodegradation is curtailed by elimination of any water leg); (3) occasionally flat, heavy API gradients where a thin oil column only partially fills the crest of the sand and an extensive OWC-area underlies the entire thin-oil column; and (4) complexly non-linear gradients with heavy oil at both the top (possibly due to up-dip top water) and again near the base towards the OWC, but relatively better oil quality in the mid-column in between (shallowest Western West Sak). All API gradients appear to be accompanied by GOR and solution-gas compositional gradients as well, due to increased thermogenic-gas degradation and simultaneous production of secondary microbial methane from oil biodegradation near the water zones, such that tandem API-GOR variations affect oil producibility. In addition, the productive oil columns occur in the high-quality reservoir zones (permeability: 50-1000 mD); however, tighter reservoir intervals within each column (<50 mD) appear to often contain locally heavier oil streaks, possibly marbling these oil columns with some intra-formational baffles.

AAPG Search and Discovery Article #90075©2008 AAPG Hedberg Conference, Banff, Alberta, Canada