Abstract Sandstones of the Lower Cretaceous Kuparuk River Formation comprise major reservoirs on the North Slope of Alaska. Original oil-in-place for the Kuparuk field is estimated at approximately 5 billion barrels. The Kuparuk field provides an excellent example of large scale reservoir heterogeneity created by multiple sandstone bodies. It also illustrates the control of depositional facies and diagenesis on reservoir quality. Stratigraphically, the Kuparuk River Formation is comprised of four distinct units; A, B, C, and D. Reservoir-quality sandstones are found primarily in units A and C. The A sandstone intervals, occurring within the lowermost unit, were deposited in a regressive shelf setting. In contrast, the C sandstones, present above an erosional unconformity, were deposited in a transgressive shelf setting. The reservoir in unit A is characterized by lenticular, shingled, sheet-like sandstone bodies. Average dimensions of these bodies are about 24 km (15 mi) long, 13 km (8 mi) wide, and 15 km (50 ft) thick. The best reservoir-quality sandstones in unit A are dominated by facies types indicative of episodic storm deposition. These types include hummocky cross-stratified and wave-rippled, flaser-bedded facies. Sandstone beds in these facies range from 0.1 to 1 m (0.5 to 3 ft) in thickness. Also present are facies types characterized by high mudstone to sandstone ratios. These include wavy-bedded and lenticular-bedded facies, and shale with lenticular sandstone streaks. These facies are not reservoir-quality because of high clay content and small scale, discontinuous sedimentary structures. The reservoir in unit C is characterized by a blanket-like geometry. Sandstone geometries within unit C are poorly defined because of syndepositional faulting and erosional truncations within the unit. The C sandstones are massive due to bioturbation and are highly glauconitic. The best reservoir-quality sandstones occur in the basal and uppermost intervals. Both intervals have unconformities at their base. In the case of the basal interval, this is a major erosional unconformity within the Kuparuk River Formation. These subunits are characterized by intense siderite cementation and subsequent partial dissolution. The distribution of reservoir properties is directly related to diagenesis and indirectly related to depositional facies. In summary, reservoir quality and heterogeneity in sandstone bodies within unit A are controlled by depositional processes. Lithofacies characterization is the key to understanding the lateral continuity and distribution of permeability and porosity within this reservoir unit. In contrast, post-depositional, diagenetic controls on reservoir quality are exhibited by sandstone bodies comprising the C unit. In this case, the distribution of permeability and porosity are controlled by siderite cementation and dissolution. In both the A and C units, a sedimentological approach to reservoir characterization is essential for a thorough understanding of reservoir quality and distribution of producible sandstones.

Abstract Anaerobic processes have only recently been recognized as an important mechanism in the biodegradation of crude oils. They are normally invoked to explain extensively biodegraded oils with little or no possibility of contact by oxygenated waters from an active aquifer. This work with Kuparuk Field indicates that early stages of anaerobic biodegradation can be subtle and easily missed, yet have economic impact. Kuparuk River Field, North Slope of Alaska, comprises two reservoir intervals: vertically stratified and imbricated lower shoreface sandstones (A sands), and overlying shallow marine sandstones with complex permeability structure (C sands). The vertical and lateral distribution of viscous oil (less than 20° API) shows a strong relationship to structure and faulting in the Kuparuk Field. Multiple mechanisms for the origin of tars and viscous oils can be proposed, including early aerobic biodegradation, anaerobic biodegradation, inorganic oxidation and gas deasphalting. This geochemical study, integrated with stratigraphic, structural and production data, was undertaken to help understand the origin and distribution of tar and viscous oil in the field. Obvious depletion of n -alkanes and other paraffins, classically regarded as indicative of early biodegradation, is not observed in examined samples. However, Kuparuk viscous oils show slight to extreme selective depletion in long-chain alkyl aromatic (LCAA) hydrocarbon families (e.g. alkylbenzenes and alkyltoluenes). This is interpreted as indicative of an early stage of anaerobic microbial degradation that likely destabilized the oil to promote subsequent precipitation of asphaltenes as tar. Depletions in LCAAs in core samples in the field are linked to decreased hydrocarbon/nonhydrocarbon ratio and to an increase in the high molecular weight (>C 50+ ) components of Rock-Eval 6 pyrolysates. Using a calibration curve constructed from oil Rock-Eval 6 pyrolysis, the API gravity of core oil plus bitumen can be estimated. Tar-plugged formations with depleted LCAAs have estimated API gravities <8°. Portions of the Kuparuk reservoir with higher iron content tend to show greater depletions in LCAA. Anaerobic biodegradation is likely mediated by dissimilatory iron-reducing bacteria. Biodegradation likely destabilizes the oil with respect to asphaltene precipitation such that later arrival of petroleum leads to tar in the reservoir. Increased tar and depleted LCAAS correspond to intervals with lower productivity indices, thus indicating a significant impact on petroleum producibility.

Abstract The Kuparuk River field on the North Slope of Alaska (Figure 1) is the second largest producing oil field in North America. Currently the production from the Lower Cretaceous Kuparuk formation exceeds 300,000 barrels of oil per day under waterflooding. This reservoir is overlain by the Colville, West Sak, and Ugnu reservoirs which contain an estimated 20 billion barrels of oil in place. These formations are unconsolidated, have widely varying fluid and rock properties, and will require waterflooding and enhanced oil recovery processes. Development options for all of these reservoirs include hydraulic fracturing of the injection and production wells; hence, characterization of the in-situ stress field is critical for optimizing field performance and recovery. The regional crustal stress field on the North Slope is extensional with maximum principal horizontal stress oriented northwest-southeast. Previous work on fracture direction in Kuparuk, however, indicated that the in-situ stress field was more complex in its orientation. The Kuparuk reservoir occurs within a broad northwest to southeast-trending anticline which plunges to the southeast. Normal fault patterns within the Kuparuk River field show two dominant strike trends: (1) northwest-southeast and (2) north-south. In this study the hydraulic fracture direction, at both shallow and deep horizons, was determined by integrating geologic, engineering, petrophysical and geophysical data. Dipmeter logs were processed and interpreted to determine wellbore breakout directions for both shallow and deep horizons. Formation microscanner (FMS) images were used to discriminate between incipient wellbore breakout zones and mechanical fracturing from drilling. Hydraulic fracture screenout data were correlated

Abstract The occurrence of heavy oil in the Tertiary and Upper Cretaceous sands of the Kuparuk River area, North Slope of Alaska, has been known since 1969. It was not until 1981 that delineation and development drilling for the deeper Kuparuk Formation was sufficient to demonstrate the wide areal extent of the shallow heavy oil. An inventory of these sands, conducted during 1981 and 1982, indicated that individual accumulations extended over 520 km 2 (200 mi 2 ), and that the combined estimate of oil in place could be as great as 40 billion barrels. The majority of the heavy oil occurs in two shallow intervals that are part of the Brookian marine and deltaic depositional system of the North Slope. The two informally named zones are the West Sak sands and the overlying Ugnu sands. These zones are oil-bearing primarily in the Kuparuk River and Milne Point units, where they occur at depths ranging from 610 to 1370 m (2000-4500 ft) subsea. The oil in the West Sak is a less heavy to intermediate crude with API gravities ranging from 16° to 22°. Most oil in the Ugnu sands is classified as bitumen at reservoir temperature, with API gravities between 8° and 12°. ARCO Alaska and ARCO Oil and Gas Company are currently studying the technical and economic feasibility of producing the oil in these shallow reservoirs. The West Sak is the most likely target for near-term development, because of the higher gravity of its crude oil. A pilot waterflood project to study reservoir response and drilling technology was begun in the southeast Kuparuk River Unit in late 1983. In addition, exploratory drilling for other heavy oil accumulations is planned on existing leases peripheral to the Kuparuk River and Prudhoe Bay units.