CONNECTING GEOLOGIC MAPPING TO SUBSURFACE WELL LOGS TO EXPLORE IDAHO’S FIRST PRODUCING HYDROCARBON FIELD IN PAYETTE COUNTY

The Miocene-Pliocene Payette basin in southwestern Idaho is one of the Northwest’s newest producing fields. Temperature of mature target intervals ranged from 80˚C to over 160˚C with some samples being over-mature and barren reaching over 200˚C. High regional heat flow and possibly heat contribution from basalt sills may have led to hydrocarbon generation in the area. Thermal Alteration Indices from palynomorph kerogen samples indicate that the field is in the mature wet gas to gas generation window which is consistent with reported production data. The hydrocarbons that are being produced are condensate and natural gas liquids with minor natural gas and oil that are derived from terrigenous sources.

16-2 Ma sedimentary deposits are represented by the fluvial and lacustrine Payette, Chalk Hills, and Glenns Ferry formations. These deposits provide subaqueous channel to deltaic stacked reservoir sands surrounded by tuffaceous sealing clays. Correlation of recent surficial mapping to newly available public geophysical well log data has greatly increased our understanding of the Payette basin’s petroleum components. 4200-6500 feet of sedimentary deposits are present above volcanic flows and sills in the subsurface. Speculated producing zones are between 1600-2400 feet in the Hamilton Field and 3400-4200 feet in the Willow Field. Surficial porosities of partially consolidated to unconsolidated sand reservoir analogues match geophysical log estimates with the average at 28%. Simplified depth-structure maps and isochores derived from interpreted well tops show a relative local deepening and thickening of basin sediments to the northwest of the field while the regional thickening trend is to the south towards the basin center. Roll-over structures and normal faulting that are expressed at the surface can be projected into the subsurface to create analogues of potential 3-way fault dominated traps. The operating company has acquired 3D proprietary seismic over the area, but this study only has public domain drilling data, surface indicators, and gravity-magnetic data provide a glimpse into the subsurface hydrocarbon play and basin architecture. Several marker beds across the mapping area east of the producing field have the potential to aid in the correlation of surficial to subsurface deposits.