Page 233 - Geology of Carbonate Reservoirs

214 SUMMARY: GEOLOGY OF CARBONATE RESERVOIRS

Two fields were chosen for this first section because they offer case histories that
include work done by the author and his students. First - hand examination of the

rock and reservoir properties in these fi elds confirmed that these examples are suit-
able for inclusion in the rather rare category of depositional reservoirs because
diagenetic alteration of pore geometry and type has been nominal. The examples
are North Haynesville Field in Louisiana (Jurassic Smackover Formation) and
Conley Field in Texas (Mississippian Chappel Formation). Production at North

Haynesville Field is from slightly modified interparticle porosity in ooid – peloid –
rhodolite grainstones (Bishop, 1968 ; Ahr and Hull, 1983 ). Production at Conley Field
is from slightly altered intraparticle porosity in fragmented, crinoid – bryozoan grain-
stones and packstones (Ahr and Walters, 1985 ). In both examples, the depositional
facies boundaries closely correspond to the outlines of underlying paleostructural
highs. The paleostructural feature at North Haynesville Field is a salt anticline that
has changed shape since the time of Smackover deposition such that part of the
present structural crest has moved. It no longer conforms exactly to the shape of
the grainstone buildup that originally formed on the crest of the salt dome. Skeletal
grainstones at Conley Field were deposited on the crest of an anticedent high inter-
preted to be a horst block that has undergone several episodes of vertical movement
accompanied by moderate erosion. In the Conley Field example, present structure
closely conforms to the outlines of depositional reservoir bodies indicating that
present structure is a reliable guide for determining drilling locations — a fi rst step
in the development of a geological concept for exploration and development. In the
North Haynesville example, the outlines of the depositional reservoir and present
structure are slightly offset but interval isopach maps reveal the shape of the original
salt structure that corresponds more closely to depositional facies boundaries.

8.3.2.1 North Haynesville Field

Location and General Information North Haynesville is one of several Smackover

fields in North Louisiana associated with faulted salt anticlines (Figure 8.1 ). Our
study began after this field had been developed so that a large amount of geological

data was available. Borehole cores provided us with an excellent opportunity to
identify genetic pore types (interparticle in ooid – peloid – rhodolite grainstones and
packstones), wireline logs enabled us to correlate stratigraphic intervals and tops
with which to map the reservoir body (an elongate grainstone – packstone sand wave,
or tidal bar complex, lying on the paleocrest of a faulted salt ridge), and core analy-
ses along with data from wireline logs enabled us to describe reservoir fl ow units
by their porosity and permeability characteristics.

Structural Setting The present structure at North Haynesville Field (Figure 8.2 )
was mapped by Bishop ( 1968 ) as an elongate, faulted anticline. Our remapping from

wireline log data confirmed Bishop ’ s interpretation. Faulted anticlines are typical in
the salt basins of the Ancestral Gulf of Mexico. In the East Texas, North Louisiana,
and Mississippi salt basins these halokinetic structures produce a variety of struc-
tural shapes and sizes depending on the thickness of salt beneath the structure, the
distance from the updip edge of the salt, the amount of overburden, and the length
of time the structure had been growing. Salt structures are known as “ salt ridges,
pillows, and domes ” depending on the thickness and shape of the structure. Salt