Effective
approaches to improving oil recovery from any reservoir must be
based on an accurate model of the reservoir framework. Key to the
construction of the framework is a detailed understanding of the
sequence architecture, facies, cyclicity and facies-stacking, and
diagenesis. Although some of these data are obtainable from subsurface
data (cores and wireline logs), accurate reservoir frameworks must
be based on outcrop models.
To develop the
most accurate model of reservoir architecture in the Fullerton Clear
Fork field, we conducted detailed outcrop studies of reservoir-equivalent
deposits in the Sierra Diablo of West Texas (Figure 3). The sequence
architecture (Figure 4) defined from these outcrop studies provides
a crucial basis for sequence-based reservoir architecture. Key elements
of this architecture are (1) a basal Leonard (L1) sequence characterized
by top-lapping, clinoformal outer platform fusulinid-rich wackestones,
(2) karst-related cave and sinkhole formation, sediment infill,
and collapse associated with exposure at the L1-L2 sequence boundary,
(3) three high-frequency sequences in the lower Clear Fork (L2)
characterized by upward-shallowing successions of outer to inner
ramp facies, and (4) a siliciclastic-rich, basal transgressive leg
of the L3 sequence (equivalent to the Tubb unit in the subsurface).
Interpretation of subsurface data in light of outcrop models shows
that the sequence architecture at Fullerton field (Figure 5) is
very similar to that expressed in outcrop.
Outcrop studies
also provide information on styles of facies- and cycle-stacking,
porosity distribution, and cycle continuity that are crucial for
identifying such patterns in the subsurface. A key observation from
outcrop studies is that cycles (Figure 6) are typically composed
of basal, lower porosity, mud-rich facies and capping, grain-rich
facies having higher porosity. Core studies (Figure 7) reveal that
these relationships among facies, cyclicity, and porosity are key
features of the reservoir architecture and that these relationships
form the fundamental basis for correlation and development of a
cycle-based reservoir framework. |