Petrographic and microfacies analysis of the Shublik formation, northern Alaska: implications for an unconventional resource system

Abstract

The North Slope of Alaska includes a world-class conventional petroleum system that has been producing for approximately 40 years. While the regional stratigraphy includes multiple proven source and reservoir rocks, the Middle to Upper Triassic Shublik Formation (Fm.) is the most prolific. The decline in conventional hydrocarbon production on the North Slope and the presence of high quality source rocks inspire the evaluation of the Shublik Fm. as an unconventional petroleum system where oil or gas are produced directly from source rocks. The Shublik Fm. is a heterogeneous calcareous shale and limestone interval that has been interpreted to indicate deposition influenced by marine upwelling. Lithofacies observed in outcrop consist of intervals of non-resistant organic-rich packages that are interbedded with resistant coarsening upward rhythmic depositional successions of phosphatic and carbonate parasequences. The heterogenous characteristics of the microfacies is one of the challenges in the unconventional exploration of the Shublik Fm. Detailed microfacies descriptions, as part of this study, document the complex lithologies and identify patterns in the occurrence of the microfacies. Microfacies descriptions are the building blocks to identify stacking patterns that define the parasequences. The parasequences observed in core, outcrop, and petrographic analysis are calibrated to well logs to map relevant stratigraphic intervals on a regional scale. A targeted interval for the unconventional exploration of the Shublik Fm. is identified using a relative brittleness index. Flat clam and phosphatic parasequences are the primary packages present within the targeted interval. The parasequences are comprised of small-scale brittle and ductile couplets. High resolution pore imaging and mechanical stratigraphy characteristics are documented within the interval. Pore types and networks provide potential storage and migration pathways within unconventional resource systems. Pore types were imaged at a high resolution using scanning electron microscopy. Pore types that are present within the Shublik Fm. include interparticle, intraparticle, porous floccules, moldic, microfracture, and microchannel. The phosphatic limestone facies contains the greatest amount of porosity including interparticle, intraparticle, and moldic within the phosphate nodules and matrix and fractures surrounding phosphate nodules. Microfracture, microchannel, and intraparticle porosity are the primary pore types within the flat clam facies. Mechanical stratigraphy has a direct impact on the success of hydraulic fracturing. The microfacies descriptions identify laminae scale mechanical stratigraphy characteristics which include erosional surfaces, laminations, graded bedding, mineralogical variation, and textural anisotropy and isotropy. A regional correlation of the targeted interval was developed building upon previous work and utilizing sequence stratigraphic models. Significant parasequence surfaces are readily identified in the well log signatures. The challenge of the heterogeneity of the Shublik Fm. is simplified first by identifying a targeted interval consisting of brittle and ductile packages enriched in TOC. Then recognizing the parasequences contained as part of the interval which include significant characteristics pertaining to pore networks and mechanical stratigraphy. Combining methods and results of this study provides an important component to evaluating the Shublik Fm. as a potential unconventional resource system.