Diagenesis and Reservoir Analysis of the Birdbear Formation, Williston Basin, North Dakota.

摘要

The southwestern part of Birdbear Formation of the Williston basin is part of the three shelf-wide sedimentary cycles in the Devonian transgressive-regressive sequence rock deposit. It is bounded above by the Three Forks and below by Duperow Formation. The formation can be informally divided into an upper A zone and a lower B zone. The common rock types in the Birdbear are dolomites, limestone, and anhydrite as well as combinations of anhydrite limestone, and dolomite limestone that is mostly found in the upper zone usually at the contact between the Three Forks and the Birdbear. Hydrocarbon has been found in the A zone in a stratigraphic trap formed by the anhydrite acting as a seal and in the B zone by the structural trap caused by the dissolution of salt in the Prairie Formation.;The formation has undergone different diagenesis which has altered its primary sediments and giving rise to secondary porosity and permeability. The formation contains mostly wackestone, packstone, and biosparite. Dolomitization and fossil fragments dissolutions are the two most common diagenetic processes occurring in the Birdbear, hence creating intercrystalline and biomoldic porosities. The reservoir quality analyses show that the Birdbear Formation has decent porosity and permeability but the answer to why the Birdbear is not a prolific producer can be found in the TOC/Rock-Eval analyses.