Bakken Formation > Geology >
The upper and lower Bakken members are similar in that they are hard, siliceous, pyritic, fissile, brownish black to dark
black, often noncalcareous, and organic-rich shales.3 The shales are very uniform throughout their extents; however, some
cores of the lower shale have thin (3 feet) black limestone that is coarsely crystalline and significantly lower in organic content.
The middle member’s lithology is highly variable and consists of five distinct lithofacies. The unit is well-sorted
(although bioturbation is common); well-cemented with calcite, dolomite, or silica; and light to medium gray in color.
All lithologies within the middle member have low primary permeability and porosity.
Middle Member Lithofacies 4
– SILTSTONE, massive, dense, mottled, dolomite, argillaceous, tan to light gray, fossiliferous, disseminated
pyrite, slightly bioturbated; contact with upper member is sharp. Cross ripple laminations, as much as 6 inches
thick, occur lower in half of section (occasionally fossiliferous).
Lithiofacies 5 (PDF, 47 MB)
– LOWER ARGILACEOUS PACKSTONE TO FINE-GRAINED SANDSTONE AND UPPER INTERBEDS OF DARK GRAY SHALE AND BUFF
SILTY SANDSTONE. Lower portion is medium gray, very fine-grained, dolomite cement, with thin parallel laminations. Upper
portion is thinly laminated to cross-bedded siltstones and very fine-grained sandstones, with moderately bioturbated, vertical
burrows. Dolomitic cement in upper and lower portions with some dewatering structures. Lithiofacies 4 (PDF, 37.8 MB)
– DARK GRAY, FINE-GRAINED SANDSTONE TO MEDIUM GRAY LIMESTONE. Sandstone is mainly quartzose with minor
feldspar and heavy minerals, buff to green, moderately well-sorted to well-sorted, rounded to well-rounded. Contains calcite
cement with occasional pyritic cement, disseminated pyrite, few brachiopods, and slight to no bioturbation. Limestone occurs
along southwestern extent and ranges from sand–algal packstone, to ooid packstone, to an ooid–crinoid packstone–grainstone.
Oil staining may be present in the very fine-grained, laminated, predominantly quartz sandstone portion. Lithiofacies 3 (PDF, 47 MB)
– GREENISH GRAY ARGILLACEOUS SILTSTONE TO BROWNISH GRAY SILTY SANDSTONE. Siltstone is moderately to very
strongly bioturbated (disrupting the laminae), fossiliferous, dolomitic becoming calcareous with depth, and disseminated
pyrite. Sandstone accumulations are restricted to localized highs. Lithiofacies 2 (PDF, 66.9 MB)
– ARGILLACEOUS SILTSTONE, massive, dense, mottled, very calcareous, gray-green, highly fossiliferous,
with disseminated pyrite. The lower contact may be either gradational or erosive. Lithiofacies 1 (PDF, 41.7 MB)
Lithiofacies Central Basin Facies (CBF) (PDF, 54.7 MB)