Geologic reservoir characterization of the Codell sandstone: central Wattenberg field, Colorado

Abstract

Colorado’s Wattenberg Field is a giant natural gas and oil field. Since its discovery in the 1970’s, the field ranks as the seventh largest domestic natural gas field in the United States, with cumulative production according to IHS 210 MMBo and 4.03 Tcf. The Codell serves as an important and highly productive reservoir in the area. The Late Turonian Codell Sandstone Member of the Carlile Shale is a zone which has produced almost exclusively from the Wattenberg Field since production from the formation began in 1981 with little to no success elsewhere in the basin until recent years. With advancements in multi-stage hydraulic fracturing and horizontal drilling there has been renewed interest in the development and continued exploration of the Codell Sandstone. This study is meant to focus on the geologic reservoir characterization of the Codell Sandstone in Wattenberg Field, and understand the future potential for the Codell play that remains untapped. The Codell Sandstone within Wattenberg Field is the low porosity (4-12%), low permeability (<0.1 mD) member of the Carlile Shale Formation. The sandstone was deposited on a shallow marine shelf below normal fair-weather storm wave base under low energy conditions. The Codell is the upper member of the Carlile Formation and unconformably overlies the lower Carlile Shales or the Greenhorn Marlstones and is unconformably overlain by the Fort Hays Member of the Niobrara Formation. Within the study area the Greenhorn and Niobrara Formations are considered to be the main source beds for the Codell hydrocarbon accumulations. Throughout Wattenberg Field the Codell Sandstone contains three distinct facies. The first of which is highly bioturbated and thick (7-15 ft), and is found at the very top and very bottom parts of the sandstone and encapsulates the other two facies. Facies #2 is thinner (1-10 ft) containing clay laminae with sand-filled burrows, and is found in the middle part of the Codell. The third facies, facies 3, is the most distinct of the three with low angle (hummocks) cross-stratification. In total the three facies of the Codell are on average 10 to 30 ft thick. High clay content, sometimes exceeding 25%, within the Codell Sandstone lead to the low permeability and low resistivity observed. Within the study area the Codell is classified as a “tight” oil and gas sand, with a dual porosity system. Hydrocarbon entrapment in the Codell Sandstone is due to a combination of low permeability, stratigraphic compartmentalization, structural compartmentalization, and clay smearing in faults. This makes the Codell play a combination play, with areas of continuous accumulation along with some areas of discontinuous accumulations. High temperature and pressure anomalies, as well, coincide with increased production rates within the Central Wattenberg Field. The future of the play will see increased horizontal drilling on closer spacing, and more precisely designed hydraulic fracture stimulations with multiple stages. In the long run there will be lower drilling and completion costs, which will make the play as profitable as possible. The culmination of this entire study can be a basis for future exploration around the world. Identifying tight, bioturbated, shelf-deposited sandstones and using the conclusions drawn from this study to aid in their economic success.