Diagenesis, Porosity and Petroleum Source Beds of Birdbear Formation, McKenzie County, Williston Basin, North America
Abstract
The Birdbear Formation in the Williston Basin is a carbonate-evaporite
sequence that extends across western North Dakota, eastern Montana, southern
Manitoba and southwestern Saskatchewan. Approximately 180 vertical and
horizontal Birdbear wells have cumulatively produced over 25 million barrels
of oil equivalent in North Dakota, and Birdbear production also extends into
eastern Montana and southern Saskatchewan. This study evaluates the degree
of diagenesis and its effects on porosity and permeability, as compared to the
depositional patterns/facies that enhanced the reservoir characteristics of the
Birdbear Formation in McKenzie County North Dakota, in relation to its
hydrocarbon potential in the Williston Basin of North America. This study
examined: 1) core and thin
section
(Thirteen cores and over 50
thin
sections);
2) geochemical
analysis
of prospective petroleum source beds (TOC Rock-
Eval program pyrolysis); 3) wireline log correlations and porosity distribution
mapping; 4) compiled standard core-plug porosity-permeability measurements
and 5) nuclear magnetic resonance (NMR) spectroscopy measurements.
The aim of the study was to determine the role of deposition and
diagenesis on porosity types, lateral and vertical distribution of reservoir
porosity, lateral and vertical distribution of prospective source beds and
anhydrite seals.
The Birdbear has two distinct units; 1) a lower carbonate dolomitelimestone
(boundstone to wackestone) unit, with discontinuous dolomitized
reservoir and abundant organic material that is being evaluated as potential
source material for hydrocarbon production using TOC Rock Eval Pyrolysis;
and 2) an upper anhydrite-carbonate (packstone) unit that has the ability to
entrap migrating fluids within the study area. Rocks in the upper
section
of the
lower carbonate unit were exposed to high diagenetic alteration that enhanced
permeability and porosity types that includes, intra-crystalline, inter-granular,
with the most dominant being solution enhanced/ moldic vuggy porosities.
These were confirmed by
thin
section
analysis
that revealed varying degree,
types of porosity and NMR spectroscopy
analysis
from marked producible
fluids indications and porosity maps across McKenzie County (center of the
Williston Basin).
The rocks in the formation showed marked selective or partial
dolomitization, micritization and dissolution of calcite cement from inclusion
of brines, creating excellent reservoir rock qualities of porosity and
permeability (average limestone porosity at 1.74%, from a range of 0.48% to
2.26% and average dolomite porosity of 8.69%, from a range of 0.86% to
29.85%, with permeability range of 20-85 md). Hydrocarbon production in the
Birdbear Formation has been documented to be from restricted intervals of
varying lithological thicknesses of 1-2 foot thick reservoirs in the upper
Birdbear, with 10+ foot thick reservoirs within the lower “B” porous zone,
which are more prominent reservoirs for production. TOC Rock-Eval program
pyrolysis data indicate fair to good distribution of prospective source beds
(TOC range from 1.06% to 11.5%, with a maximum HI value of 182.8 and OI
value of 11.9) that are thermally mature and within oil generating window with
Tmax value of 452. This indicates the Birdbear Formation may be a selfsourced
hydrocarbon system within the Williston Basin.
AAPG Datapages/Search and Discovery Article #90357 ©2019 AAPG Rocky Mountain Section
Meeting, Cheyenne, Wyoming, September 15-18, 2019