Diagenesis of the middle member of the Bakken Formation, Williston Basin, North Dakota.

摘要

The shales of the Bakken Formation have been extensively studied since the discovery of oil, but little attention has been paid to the middle reservoir member. Successfully produced wells are concentrated in multiple fields throughout the Williston Basin, there has been no definitive way to determine a well's potential productivity. This study used core samples, point counting, and petrography to determine the diagenetic traits common to the Middle Member of the Bakken Formation and their effect on oil production. Cores from forty-one wells were examined. Thin sections were made from sixteen cores, eight producing wells and eight dry wells. All wells were used in the construction of cross sections and isopach maps.;The point counts used all traits common to each well with thin sections: calcite, dolomite, fossil/fossil fragments, micrite/micritic matrix, porosity (intragranular and fracture), pyrite, quartz silt, and any miscellaneous grains not otherwise taken into account. All cores were analyzed for other potential diagenetic traits such as compaction, soft sediment deformation and soft sediment structures, burrows, and residual hydrocarbons. Cross-section work involved creating "beds" of four commonly shared traits and tracing the highest recorded occurrence and lowest recorded occurrence within the well. The "trait beds" were inserted in a stratigraphic column of members of the Bakken Formation to compare their structure to the Bakken Formation over the course of the cross section. No distinct patterns were revealed though a larger sample of wells and cores may provide different results.;Of all the selected traits, pyrite, quartz silt, micritic matrix, and porosity have the greatest influence on oil production. Pyrite growth is restricted to between 4% and 8% in producing wells while varying widely in dry wells. The reason is unknown though it is strongly suspected that it is directly related to the chemistry of the basin water flowing through producing oil fields. Quartz silt percentages are highest in producing wells and may provide a strong, open framework for fluids to flow through. Micritic matrix is significantly higher (twice as much) in dry wells than producing wells, with approximately one-third of the rock composed of micrite. The fine-grained clay and mud most likely reduces porosity and permeability to nonproducable levels. Lastly, total porosity is extremely important and appears to rely more often on fracture porosity than intragranular porosity. Porosity is almost always higher in producing wells, but rarely over 5% of the total rock. A project using a significantly larger sample size of thin sections would provide clearer patterns of diagenetic traits. Analysis of water chemistry may also provide answers to questions about pyrite and other authigenic mineral distribution, as well as fluid migration.