Sedimentologic, Stratigraphic and Architectural Attributes of the Iles Formation, Northwest Piceance Basin, Rangely Colorado

Caldes, Brice A.
Aspect Resrouces, Denver, CO

Upper Cretaceous continental strata of the Iles Formation were chosen to highlight the systematic changes in numerous sedimentologic and stratigraphic attributes important for reservoir characterization. Predictable changes in facies successions and associated stratigraphic and sedimentologic attributes define unidirectional trends of the accommodation:sediment supply (A/S) ratio. These trends in the A/S ratio are recorded by: facies associations, successions and diversity; macroform types, associations and diversity; the degree of preservation of original geomorphic elements; and large-scale stratigraphic architecture.

Channelbelt sandstones deposited under low A/S conditions are more laterally continuous (>1 km wide in depositional strike orientation) and have good internal sandstone connectivity. They have low facies and macroform diversity and low mean trough cross-stratification (TRX) set thickness. Conversely, high A/S channelbelt sandstones are isolated and laterally discontinuous (200-300 m wide in depositional strike orientation). They have high facies and macroform diversity, and higher mean TRX set thickness. Crevasse splay strata also display accommodation-related facies differentiation and architectural dissimilarity. Low A/S crevasse splays have a low facies diversity, and are composed predominantly of thin (1-2 m), low-angle beds of ripple-laminated sandstone. High A/S crevasse splays are thick (5 m) and display high-angle (10°-15°) bedding with increased facies diversity.