James R. Markello
ExxonMobil Upstream Research Company
Fluid flow in the Wahoo Reservoir of Lisburne Field is differentially controlled by matrix reservoir quality, reservoir architecture, and overprinting of faults and fractures across the field. To understand and predict specific fluid flow pathways and field performance, the multi-company team completed an updated reservoir characterization with a new depositional model, new sequence stratigraphic-based reservoir architecture, and new fracture assessment by reinterpreting cores, logs, and seismic. The new interpretations were the basis for populating new sector and full-field geologic models with matrix and fracture properties. These scaled geologic models were dynamically simulated and achieved reasonable performance history matches.
The multi-billion barrel OOIP Lisburne Field (27.5km max length; 14.2km max width; ~360km2) was discovered in 1968 by the Prudhoe Bay State #1 well. The field is an oil accumulation (27°API gravity) with gas-cap, and is operated by BP with ExxonMobil, ConocoPhillips, and Chevron as majority working interest owners. It consists of the Wahoo (Early Pennsylvanian) and Alapah (Late Mississippian) mixed carbonate-clastic-evaporite reservoirs. To date, six delineation wells and 95 development wells have been drilled from six surface pads (L1, L2, L3, L4, L5, and LGI-1). Field production is by pressure depletion and gas-cap expansion. First oil was produced in 1985. Field production peaked in 1988 at about 47KBD and declined to under 10KBD by 1997. Since then, production has varied from 5 to 13KBD. Well tests, spinner surveys, and temperature logs have been run in various wells, and tracer experiments were conducted between wells of the L2 drilling pad. L2 pad was the location for a water injection pilot from 1988-90. Water injection pilots commenced in 2011 for pressure maintenance and secondary recovery in the L3 and L5 pads.
The updated 2009-2011 reservoir description included new depositional, diagenetic and stratigraphy syntheses that incorporate 1) known global-scale geologic constrains of Early Pennsylvanian-age Pangean-assembly tectonics, paleogeography-directed warm vs. cold-water ocean circulation, climate-driven high-amplitude high-frequency icehouse eustasy, and 2) concepts of sequence stratigraphy, the stratigraphic hierarchy, syn-tectonic differential subsidence on an actively deforming distally-steepened ramp, mixed carbonate-clastic depositional facies systems with reciprocal and coeval sedimentation styles, and tectonic-driven surface and burial episodes of diagenesis. Four key sedimentologic problems required solutions before the new sequence stratigraphic architecture could be constructed. The focus of the talk will be on these four problems and how the answers were used in the construction of the new reservoir architecture. These include: 1) How did the complexly mixed carbonate-clastic depositional system of the Wahoo Formation work? 2) How can strata and facies composed of warm-water ooids, coated-grains, and photozoan faunal grains be interbedded with strata and facies composed of cool-water heterozoan faunal grains? 3) On a “low angle ramp” how do exposure surfaces observed in one well relate to “coeval” lower shoreface to open shelf strata in adjacent wells? And 4) Why are there no reefs or carbonate buildups of any kind present in the Wahoo Formation?