Arthur H. Saller¹, Brian Ball², Steve Robertson²

(1) Unocal Corp, Sugar Land, TX
(2) Unocal/ Pure Resources, Midland, TX

ABSTRACT: Reservoir Characteristics of Devonian Cherts and their Control on Oil Recovery: Dollarhide Field, West Texas, USA

Approximately 70 million barrels of oil have been produced from the chert-dominated Thirtyone Formation (Devonian) at Dollarhide Field in west Texas. The chert reservoir contains approximately 83% of the original oil in place, and consists of two different facies - laminated microporous chert and burrowed chert. The laminated microporous chert was deposited as sponge spicule sands (grainstones) in channels and fans on the slope of the Tobosa basin. The burrowed chert facies was deposited as burrowed mixtures of sponge spicules, siliceous mud and carbonate mud in broad slope environments between fans and channels. Spicules came from the disaggregation of siliceous sponges living on the slope. Early marine and/or meteoric diagenesis dissolved sponge spicules, resulting in spicule-moldic porosity. The burrowed chert underwent small-scale differential compaction that produced short, discontinuous fractures.

Production is related to thickness of various facies. The porous chert varies in thickness from 0 to 24 m, and pore volume (phi-h) varies from 0 to 6 pore-meters. The laminated microporous chert is very homogenous with high porosity (25-35%) and uniform permeability (5-30 mD). Areas dominated by laminated microporous chert had moderate primary recovery (200,000 barrels of oil per well; BOPW), excellent waterflood production (1-2.5 million BOPW), and poor 20-acre infill production (<20,000 BOPW). The burrowed chert has more heterogeneous porosity (5-30%) and permeability (<1-100 mD). Wells with substantial burrowed chert had moderate to good primary recovery (200,000-300,000 BOPW), moderate waterflood recovery (300,000-1,100,000 BOPW), and moderate 20-acre infill recovery (50,000-100,000 BOPW).

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado