Talk: Depositional Environments and Stratigraphy of Valley-Fill Deposits in the Lower Cretaceous D Sandstone (Cenomanian), Denver Basin, Colorado
Speaker: Steven M. Goolsby
Location: Marriott City Center
Date: July 2, 2010
Abstract: Published in The Outcrop, July 2010, p. 22-23
Valley-fill deposits constitute an important producing reservoir for the lower Cretaceous D Sandstone in the Denver Basin of Colorado. These deposits are typically one-half to a mile or more in width and tens of miles long in plan view. Barrier and shoreline-associated “marine bar” deposits are often located sub-perpendicular to these valley-fill deposits. The resulting depositional patterns led to early interpretations of fluvial or distributary channel depositional environments for the valley-fill deposits. Recent studies have demonstrated that the valleys are actually filled with complex estuarine facies. Abundant tidal and biogenic sedimentary structures that are not indicative of fluvial sedimentation can be identified in cores for this interval. Bay-head delta, proximal bay fill, distal bay fill, and estuary mouth facies can be identified in cores. Estuarine deposits in Kouchibouguac Bay, New Brunswick, Canada, serve as a modern analog to the D Sandstone valley-fill deposits. A depositional sequence in which low-relief valley topography was back-filled during an overall marine transgression is proposed for the D Sandstone “channel” system. Within the overall transgression, back-stepping due to lower-order sea level fluctuations resulted in complex stacking patterns in the valley fill, often juxtaposing proximal bay head and distal bay mouth deposits next to each other. For example, proximal clastic-rich bay head delta deposits may be stacked over outer bay-mouth bioturbated shale-rich deposits locally. The complex stratigraphy that results from this stacking pattern adds considerable risk to oil and gas exploration for the D Sandstone valley-fill reservoirs.
Talk: Absorption Isotherms: Concepts, Tests, and Analysis
Speaker: Ajayendra (AJ) Kumar, Project Manager, Core Laboratories, Denver, CO
Location: Marriott City Center
Date: July 16, 2010
Abstract: Published in The Outcrop, July 2010, p. 23
Sorption isotherm data is of a primary importance for Gas-in-Place calculations as well as the assessment and prediction of performance of an unconventional gas reservoir. As the reservoir pressure decreases, the capacity of a CBM well to desorb methane goes down as well. This phenomenon can be very accurately measured in laboratory conditions using precise measurement and computing equipment. The data, in the form of a plot can predict the volume of gas that will be released with continued production. The storage capacity of coal to hold gas is in most cases a function of rank of the subject. This technology has been extended to perform isotherm tests on shales. While the gas storage capacities on shales can be much smaller as compared to coals, the correlation between geochemical properties and storage capacities provides good evidence that this data is as pertinent in shale reservoirs. A typical canister desorption data combined with an adsorption isotherm gives a very accurate representation of how shale gas reservoir will perform with continued production. Multiple gas isotherm tests are recommended if the percentage of certain gases other than methane is detected in gas composition analysis during the canister desorption process. Mixed gas isotherms are modeled using IAS (ideal adsorbed solution) theory of Myers and Prausnitz. Mixed gas isotherms are of major importance in ECBM (enhanced CBM) industry. This talk provides basic concepts of adsorption, how the isotherm tests are performed in a laboratory and their importance.