Yates Formation of North Ward-Estes and South Ward Fields, Permian Basin of Texas-New Mexico: Part II: Clastic Reservoir Characterization
DRONAMRAJU, S., R. JOHNSON, W. M. AHR, J. M. MAZZULLO, and T. A. BLASINGAME
This study describes the petrography and reservoir properties of clastic
facies in the Yates Formation on the Central Basin Platform of the Permian
basin. The Yates is a sequence of carbonates, clastics and evaporites which was
deposited in fluvial, deltaic
, eolian, sabkha, and shallow marine environments.
It contains two highfrequency sequences (HFS) and 18 parasequences, each of
which is characterized by an "upward-drying" transition from shallow marine to
continental facies.
The best quality Yates reservoirs
are found in the well sorted, fine-grained
fluvial-
deltaic
sandstones which are preserved in the IVFs at the bases of the
HFSs; in the thick eolian sandstones within the prograding units at the tops of
HFSs; and in the thin eolian sandstones within aggrading and retrograding
parasequence sets. The permeabilities (k) of these sandstones range from 1 to
250 md and their porosities (phi) range from 5 to 27%. The porosity is largely
secondary in origin, formed by the dissolution of intergranular cement and
feldspars. The poorest quality
reservoirs
, with phi = 1% and k < 0.2 md, are
found in the dolomite- and evaporite-rich clastics which accumulated in sabkhas
and coastal environments. The anhydrite-cemented sabkha facies and subtidal
carbonates act as vertical flow barriers.
Oil production from the clastic reservoirs
is characterized by early
depletion and water influx, leaving 60% of the oil unrecovered. Careful
selection of perforation intervals and selective isolation can improve the
recovery by suitable flooding mechanism, even in mature field area. This study
is expected to serve as a model for flooding Yates
reservoirs
on a larger scale.