Effects of depositional environments and thermal maturity on source and reservoir characteristics –

Effects of depositional environments and thermal maturity on source

and reservoir characteristics –

A case study of Upper and Lower Bakken “shale” cores, Williston Basin, North Dakota

Laura L. Wray

                        The upper Devonian lower Bakken “shale” and lowermost Mississippian (Kinderhookian) upper Bakken “shale” members of the Bakken Formation are major hydrocarbon source rocks in the Williston Basin of North Dakota and Montana.  The term “mudstone” is a better descriptor than “shale” because these source rocks are not clay-rich, laminated, fissile, detrital sedimentary rocks.  Rather, they are better described in this study as  massive, dark-brown to black, fine-grained, organic-rich, silty, fossiliferous mudstones. Data from six cores analyzed along a 52-mile east-west transect in McKenzie and Mountrail County, ND, provide a rich suite of information for regional comparisons of stratigraphy, reservoir characteristics, depositional environments, and thermal maturity.

          Overall thicknesses in both intervals are variable across the transect.  Visual inspections of these mudstones fail to show many subtle compositional and textural components.  Both the upper and lower Bakken contain detrital silt that is disseminated throughout, seen only in thin sections and SEM images, as well as occasional thin, locally discontinuous silt laminae that are visible in the slabbed cores.  These eolian silt grains, along with a variety of pelagic, biogenetic quartz fossil fragments and fecal pellets, settled through a stratified water column and accumulated on a mostly anoxic seafloor.

          Mineral assemblages identified in thin sections and by SEM images are surprisingly diverse.  X-ray diffraction results show that the major components that are relatively constant along the transect include: detrital, biogenic, and authigenic quartz (30-50% for upper Bakken, 30-45% for lower Bakken); predominantly Type II kerogen (10-20 wt. % for upper, 11-18 wt. % for lower); illite/mica (15-25% for upper, 13-29% for lower); and K-feldspar (4-8% for upper, 5-11% for lower).  The constituents that vary the most include: detrital and authigenic dolomite (2-13% for upper, 2-11% for lower); eastward-increasing mixed-layer illite/smectite (1-17% for upper, 3-15% for lower); authigenic pyrite (3-14% for upper, 3-9% for lower); and calcite composed mostly of skeletal fragments (1-7% for upper, 0-7% for lower).

          Present-day as well as estimated original TOC values highlight the well-documented westward increase in thermal maturity parameters for both source rocks with a few interesting exceptions close to the Nesson Anticline.  Corresponding organic pore development appears to be related in increasing thermal maturity.

          Chemostratigraphic interpretation of key elements from hand-held X-ray fluorescence demonstrates trends within both mudstones that characterize depositional and diagenetic variations, allowing several sequences to be defined that otherwise would not be viewed visually or with other analyses.  These sequences can be related directly to sea level changes and accompanying water column chemistry, upwelling, and detrital input. 

Speaker Laura Wray
          Laura L. Wray is a temporarily retired petroleum geologist with three decades of conventional and unconventional reservoir development drilling and completion (including oil and gas “shales”), horizontal well planning, exploration work, prospect evaluations, and coalbed methane (CBM) research.  Additional technical and business skills include core analysis, ...

          Laura L. Wray is a temporarily retired petroleum geologist with three decades of conventional and unconventional reservoir development drilling and completion (including oil and gas “shales”), horizontal well planning, exploration work, prospect evaluations, and coalbed methane (CBM) research.  Additional technical and business skills include core analysis, petrography, expert testimony, technical editing,  teaching/training/mentoring, organizing and leading geologic fieldtrips and seminars, publishing technical and statistical reports on Colorado's petroleum plays, and geologic recruiting.  She has worked for Amoco Production Company, Williams Production Company, WPX Energy, the Colorado Geological Survey, the U.S. Geological Survey, and as a consultant to the BLM. She has been an active volunteer with the Rocky Mountain Section of Geologists (RMAG) in Denver, AAPG, the Rocky Mountain Section of AAPG, and the American Institute of Professional Geologists.

Full Description
Organizer WTGS

When?

Tue, April 7, 2015
11:30 a.m. - 1 p.m.
(GMT-0600) US/Central

Event has ended

Where?

Midland Center
105 N. Main Street
Midland, TX 79701
United States