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Working toward a Stable Stratigraphic Reference Framework for Oligo-Miocene Fluvial Sequences in Cook Inlet, Alaska: Chemostratigraphic Characterization and Correlation of the Hemlock, Starichkof and Tyonek Formations

Ratcliffe, Ken 1; Wright, Amelia 1; Kremer, Meg 3; Dix, Michael C.4; Hughes, Simon N.4; Reed, Tim 2; Jackson, Tavia 2
1 Chemostrat Inc, Houston, TX.
2 Pioneer Natural Resources, Anchorage, AK.
3 Alaska Department of Natural Resources, Division of Oil and Gas, Anchorage, AK.
4 Halliburton - Sperry Drilling Services, Houston, TX.

Cook Inlet is a prolific gas-prone hydrocarbon basin that has been the subject of exploration efforts since the 1950s. Despite decades of study, the complexity of the Oligocene-Miocene, fluvial-dominated depositional systems have prevented the establishment of a stable reference stratigraphic framework. Traditionally, for petroleum basins, such frameworks rely on lithostratigraphic principals or derivatives thereof, such as wireline log correlation. In Cook Inlet, however, the thick, homogeneous and fluvial-dominated nature the basin-fill has resulted in several, often apparently conflicting, lithostratigraphic schemes. This lack of a stable reference stratigraphic framework hampers effective hydrocarbon exploration and exploitation.

Here, variations in whole-rock elemental (inorganic) composition of cuttings samples are used to construct a chemostratigraphic-based characterization for the Hemlock, Starichkof, and Tyonek formations in four wells located in the Cosmopolitan Unit. Based on changes in values of K/Al, Cs/Al, Nb/Al, Nb/Na, Fe/Ti, Ti/Nb, and U, seven chemostratigraphic packages are defined and correlated between the four wells, thereby providing a robust stratigraphic reference framework within the Cosmopolitan Unit. These elements and element ratios are controlled by changes through time in the composition of sand-size detritus (quartz, feldspar, rock fragments and some mica), heavy mineral species and clay mineral assemblages. Initial work elsewhere in Cook Inlet suggests that it may be possible to extend this framework over much of the 200 X 70 mile inlet area, thereby providing a means to correlate on a basin-wide scale.

Although a geochemical analysis of sediment provenance indicates a mixing from several terranes throughout Hemlock-Starichkof-Tyonek deposition, variations in values of K/Al, Rb/Al, Zr/Sc, Th/Sc, Th/Co, and REE/Al, imply an overall change in the dominant sediment provenance through time. Understanding provenance changes in space and time has important implications for sand dispersal patterns, diagenetic pathways, and therefore reservoir quality.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009