Core, well log, and seismic linkage: insight into topset facies and timing of sand delivery to the basin floor in the National Petroleum Reserve in Alaska

Abstract

Lower Cretaceous clinoforms of the Brookian sequence in the National Petroleum Reserve in Alaska (NPRA) record the filling of an actively-subsiding foreland basin. The discovery of oil-charged stratigraphic traps in the NPRA, the Alpine oil field, and the Ooogoruk oil field and the emergence of Brookian topset and turbidite plays have renewed interest in the study of these clinoforms. The seismic-scale, 600 - 2000 m thick, clinoforms contain elements of a potentially significant petroleum system, including bottomset seismic facies with both source and reservoir rocks; bottomset, foreset, and topset seismic facies with reservoir rocks and trap geometries; and topset seismic facies with reservoir facies, coal, and coalbed gas resources. Core, well logs, and 2-D seismic were integrated to identify a spectrum of sedimentary facies and facies associations in seismic topsets, foresets and bottemsets and link them to basin-floor fan type and occurrence. Nearly 1100 m (3610 ft) of core and well log suites from 14 wells and a regional 2-D seismic grid covering nearly 80,000 km2 were integrated. Detailed core descriptions allowed recognition of newly-defined fluvial channel and overbank, tidal, estuarine, lagoonal, and deltaic facies associations within the seismic topsets, mostly belonging to the Nanushuk Formation. This study identifies tide-influenced, fluvial-dominated deltas that were previously interpreted as wave-influenced or even, wave-dominated. Also, a hitherto-unidentified extensive, back-stepping estuarine system is interpreted in the northeastern NPRA. Seismic foresets and bottomsets of the Torok Formation contain facies associations defined by shelf-edge deposits, upper-, middle- and lower-slope deposits, thick- and thin-bedded turbidites in basin-floor fans, and distal basin floor deposits. Six samples were analyzed for petrography to compare petrographic properties of the here identified fluvial, deltaic, and turbidite sandstone facies, which are known to be reservoir facies. This study reveals that these sandstones are lithic arenites and lithic wackes with southern and western provenances, and that depositional environment as well as post-depositional processes significantly influenced the reservoir properties of these sandstones. Fluvial sandstone is less sorted and has a greater amount of cement (mostly calcite) than more porous deltaic sandstone, while turbidite sandstone has large matrix and cement (quartz and calcite) components. Turbidite sandstone has undergone a greater degree of compaction as evidenced by brittle deformation in competent grains, plastic deformation in ductile grains, and sutured grain boundaries. The moderate- to poor-sorting and abundance of ductile grains make these sandstones more susceptible to porosity destruction by mechanical compaction. Understanding the partitioning of sediment in these systems is critical in identifying targets for hydrocarbon exploration. Through detailed core descriptions and careful definitions of facies associations, it is possible to more accurately link seismic-scale basin-floor fans to time-correlative depositional regimes on the shelf and slope. Fluvial topset facies and tide-influenced, fluvial-dominated deltaic topset facies correlate to basin-floor fans in the NPRA. Thorough analyses identified two dominant scenarios for sediment bypass in the Nanushuk-Torok depositional sequence: (1) fluvial incision at the shelf-edge during forced regressions that led to hyperpycnal flow-ignited turbidity currents that delivered sediment to basin-floor fans and (2) shelf-edge delta progradation driven by high sediment supply during relative sea-level highstands as well as falling-stages that led to eventual mass failure at the shelf-edge to upper slope and combination slump- and hyperpycnal-ignited turbidity currents that bypassed sediment to the basin floor. The former is common throughout the NPRA and the latter is limited to areas in the eastern NPRA, thus revealing a dynamic, basin-wide system controlled primarily by relative sea-level in the west and high sediment supply in areas the east. While basin-floor fan development in Lower Cretaceous clinoforms in the NPRA has been attributed to relative sea-level lowstands, linkage of detailed topset facies associations recognized in core to well logs and 2-D seismic reveals that basin-floor fans were generated during lowstands as well as highstands, depending on the timing and location of deposition within the basin.