Petrographic and Petrophysical Characteristics of the Upper Devonian Three Forks Formation, Southern Nesson Anticline, North Dakota

Abstract

The Three Forks Formation is about 230 ft thick along the southern Nesson Anticline in McKenzie County, North Dakota, and has four "benches” with distinct petrographic and petrophysical characteristics that impact reservoir quality. Clean peritidal dolomitic benches tens of feet thick are separated by more illitic (higher gamma-ray) intervals 10 to 20 ft thick. Comparing pore sizes observed in scanning electron microscope (SEM) images of the dolomite benches with the total porosity and logarithmic mean of relaxation decay time (T2 Log Mean) for 13 nuclear magnetic resonance (NMR) logs in the study area confirms that the NMR log is a useful tool for quantifying pore size distributions in the Three Forks Formation.

The top bench of the Three Forks is 35 ft thick and consists of tan to green silty and shaly laminated dolomite mudstones. Dolomite beds have good reservoir characteristics partly due to receiving the most oil charge from the overlying lower Bakken. The 13 NMR logs from the study area show that this bench averages 7.5% total porosity (8% in core). SEM study shows that intercrystalline pores and secondary moldic pores formed by selective partial dissolution of some grains are present. Intercrystalline pores are typically triangular and occur between euhedral dolomite rhombs that range in size from 10 to 20 microns. Many intercrystalline pores are partly filled with fibrous authigenic illite. Overall pore size ranges from 1 to 5 microns.

Deeper benches of the Three Forks are also silty and slightly shaly dolomites, but average pore size decreases downward through the formation due to locally abundant authigenic clays. Chlorite platelets are common in the second bench on the flank of the Nesson Anticline and reduce average pore size to 500 to 800 nm. Bound water on chlorite platelets further reduces reservoir quality and limits oil saturation to ≤20%. The third bench is the most calcareous bench with pores averaging 200 nm in size. The fourth bench contains common anhydrite nodules and is non-productive.

There are significant changes in the T2 Log Mean from bench to bench as average pore size measured on SEM samples decreases. Study of a "type" well shows that pores in the first bench have a T2 Log Mean relaxation time of 10.2 ms, whereas the 500- to 800-nm chlorite-filled intercrystalline pores in the second bench have a T2 Log Mean of 4.96 ms. This compares with a T2 Log Mean of 2.86 ms in the 3rd bench where pores average just 200 nm.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018