Reservoir analysis of the Birdbear Formation (Devonian) in northwestern North Dakota.

摘要

The Birdbear Formation of northwestern North Dakota is a carbonate -- evaporite unit deposited on a rimmed carbonate platform during the Late Devonian. The upper and lower contacts are conformable and the formation has been previously divided into two informal upper and lower members. Analyses of core intervals, thin sections, total organic carbon, and wire line logs have allowed for the identification of three informal units that are divided based on lithologic changes, depositional settings, and reservoir characteristics of the formation. Each unit is identified and mapped across the study area using core to log correlations. The units are recognized and named in ascending order as unit 1, unit 2, and unit 3.;A variable lithologic architecture is recognized within the formation, although the formation may be broken down into four primary lithologies: anhydrite, mudstone, wackestone, and packstone. Variations amongst these lithologies are identified based on the relative abundance of fossils, calcite, dolomite, and anhydrite. Unit divisions are initially recognized based on significant lithologic changes throughout the core intervals and are then coupled with changes in the identifiable reservoir characteristics to enhance the accuracy of each unit division. Analysis of the lithologic and reservoir characteristics allow for the recognition of three primary depositional settings across the platform. These settings are interpreted as subtidal, intertidal, and supratidal, which correlate with unit 1, unit 2, and unit 3; respectively. Overall, the sequence is interpreted to represent one regressive mega-cycle from the maximum transgressive deposits at the end of Duperow time, to the supratidal/sabkha conditions at the start of Three Forks deposition.;Thin section analysis of the formation presents evidence of lithological changes throughout the formation which define the available porosity within each unit. The main factors contributing to porosity changes are; anhydrite development, dolomitization, calcite crystallization, and compaction features. These processes both enhance and diminish porosity throughout the Birdbear where the primary form of porosity is intergranular pore space surrounding crystalline dolomite rhombs. Fracturing and intragranular porosities within fossil fauna locally enhance the porosity. Diminished porosity zones may be present by abundant carbonate crystallization within both intergranular and intragranular pore space. Further porosity degradation is recognized by late stage compaction of the interval, as documented by the abundant presence of stylolites and anhydrite development.;Analyses of these individual characteristics which define the Birdbear Formation allow for the analysis of the formation as a reservoir quality interval. The porosity of each unit is analyzed based on visible porosity and digitized log porosities. These analyses reveal that reservoir quality rock exists within units 2 and 3, and the potential for hydrocarbon migration through unit 1 is plausible. The presence of anhydrite within the uppermost intervals indicates the presence of a seal rock within the Birdbear interval; while TOC sampling and Rock-Eval Pyrolysis present the potential for hydrocarbon generation and migration into the reservoirs. Each of these factors suggest that the lithologic characteristics that define the Birdbear Formation are sufficient for localized production quality reservoir rock within the study area.