Geology of the Van Buren and Lavaca Quadrangles, Arkansas ...

A14 GEOLOGY OF THE ARKANSAS VALLEY COAL FIEL-PART 2
"30' 15' 94'00
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. .*. \ \ y$d//QUADF?ANGLE 0 5 10 MILES
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Well producing gas
f
Well with show of gas
EXPLANATION
Well being drilled
as of Sept. 30,1966
.
Well that did not penetrate the
mapped unit or that has in-
Well with no + show of gas
complete or unavailable
records
LAVACA
QUADRANGLE
FIQURE 6.-Map showing thickness of Morrow rocks. Thickness lines are dashed where projected;
interval is 20 feet. Numbers indicate thickness, in feet. (See fig. 11 and table 3 for
well number and location.) Stipple indicates possible reservoir area around well reported
as producing or having a show of gas; diameter is arbitrarily shown as 1 mile to agree with
well spacing.
sandy limestone, and some medium-gray siltstone. The
limestone in the upper part of the Bloyd grades laterally
to the southwest into a fine- to medium-grained limy
sandstone.
ATOKA SERIES
The Atoka Series consists of the Atoka Formation.
In this report, the Atoka Formation has been divided
into three lithologic parts which were further subdivided
into zones on the basis of electrical log characteristics.
The zones are numbered in nonconsecutive order from
1 near the top of the formation to 109 near the base
(pls. 2, 4).
The Atoka Formation overlies the Bloyd and is
unconformably overlain by the Hartshorne Sandstone.
In northern Arkansas where the contact between the
Atoka and the underlying Bloyd is exposed, it is
unconformable. In the area of this report the contact
relations cannot be determined with certainty, but the
contact is probably unconformable. In this report, the
base of the Atoka is placed at the base of the first
sandstone (base of sandstone in zone 109 or zones 108
and 109, pls. 2, 4) above beds of Bloyc-type lithology
(shale containing limestone, sandy limestone, and limy
sandstone). Light-colored bentonitic(?) shale is in or
just above the upper part of the sandstone in zone 109
or zones 108 and 109 (pl. 2, wells 158, 137, 72, 115, 50,
96, and 134). Frezon and Schultz (1961) described this
as a marker bed in the lowermost Atoka. However, in
parts of the Van Buren and Lavaca quadrangles and
elsewhere in Arkansas, the base of the Atoka is difficult
to identify because the limestone in the upper part of
the Bloyd grades into sandstone, and more than one
bed of bentonitic shale exists in the lowermost 700 feet
of the Atoka.
The amount of erosion at the unconformity between
the Atoka Formation and the overlying Hartshorne
Sandstone is uncertain. In well 36 the Atoka sequence
between the top of zone 1 and the base of the Hartshorne
Sandstone is 185 feet thick; the same unit in
well 38 (pl. 4, sec. X-X'), about 1 mile to the southwest,
is only 110 feet thick; and in well 37, about 1 mile west
of well 38 (pl. 4, sec X-X'), the unit is only 82 feet