Fracture patterns can provide insight into the strain history and stress evolution of deformed strata. In southern Alaska’s Cook Inlet forearc basin, hydrocarbon traps are typically fault-cored anticlines, where fractures likely aid in the migration of hydrocarbons from Lower Jurassic marine strata into Cenozoic nonmarine deposits. Consequently, understanding the distribution and orientation of fracture sets with respect to these structures is necessary to advance the understanding of one of Alaska’s largest petroleum provinces. This study aims to quantify the fracture intensity of four regional fracture sets, which are pervasive in deformed forearc basin strata of Jurassic age in the Iniskin–Tuxedni region of lower Cook Inlet, Alaska. We document how fracture intensity varies among the four regionally identified fracture sets: 310° (Set A), 210° (Set B), 250° (Set C), and 360° (Set D). Their respective intensities per meter for apertures greater than or equal to 0.2 mm are 6.13, 4.05, 4.75, and 5.19. Grain size and stratigraphic unit are poor indicators of fracture intensity for the rock types studied in our field area. Our analysis of the fracture sets indicates that fracture intensity is most greatly influenced by proximity to large fractures and faults. We suggest that microfracture populations can serve as fair estimators of macrofracture abundance within an order of magnitude. Future microfracture work with subsurface samples could better estimate subsurface macrofracture abundance. We conclude that natural fractures could serve as migration pathways that enhance reservoir permeability in the lower Cook Inlet hydrocarbon province, and that the 310° (Set A) fracture set is most favorably oriented for fracture stimulation under the current stress regime.

Fracture patterns can provide insight into the strain history and stress evolution of deformed strata. In southern Alaska’s Cook Inlet forearc basin, hydrocarbon traps are typically fault-cored anticlines, where fractures likely aid in the migration of hydrocarbons from lower Jurassic marine strata into Cenozoic non-marine deposits. Consequently, understanding the distribution and orientation of fracture sets with respect to these structures is necessary to improving the understanding of one of Alaska’s largest petroleum provinces. Furthermore, recent refinements in understanding southern Alaska’s Dynamic Cenozoic tectonic evolution allow us to interpret fractures in a regional tectonic context. Despite the important role fractures likely play in the Cook Inlet petroleum system, limited work exists linking fractures to regional tectonic events and structures. The objective of chapter one is to characterize from field and remote sensing observations the orientations, distributions, and relative ages of several regionally prominent fracture sets. Field observations focus on the area of the western Cook Inlet near Augustine Volcano, north to Tuxedni Bay. Remote sensing observations expand the study area from the Alaska Peninsula in the south to Mount Spurr in the north. I identified four fracture sets—with common orientations, opening modes, and relative ages—within the sedimentary sequence that spans early Jurassic to Miocene time in the Cook Inlet forearc basin. Within the field area, these sets fall into two structural domains: 1) the Iniskin Peninsula, site of an anticline–syncline pair and reverse slip on the SW-striking Bruin Bay fault; and 2) north of Chinitna Bay, where the Bruin Bay fault strikes ~N–S and preserves primarily sinistral displacement. Chapter two is aimed at quantifying the fracture intensity of the four regional fracture sets defined in Chapter 1, which are pervasive in deformed forearc basin strata of Jurassic age in the Iniskin–Tuxedni region of the lower Cook Inlet, Alaska. I document how fracture intensity changes between the four regionally identified fracture sets of chapter one. Analysis of fracture intensity indicates that changes in fracture intensity are guided by the opening of other fractures and grain size. I also measured fractures at the thin-section scale, via back-scattered electron microscopy, to test the feasibility of using micro fracture analysis to estimate macro fracture abundance. I conclude by discussing how natural fractures could enhance sub-surface permeability for the lower Cook Inlet hydrocarbon province; and serve as migration pathways in the lower and upper Cook Inlet petroleum systems.

20 papers on sedimentary environments ranging from Paleozoic to Holocene covering both onshore and offshore Alaska.

The purpose of this paper is to report briefly on ongoing structural, thermochronologic, and stratigraphic investigations along the northwestern margin of Cook Inlet basin.