Page 161 - Geology of Carbonate Reservoirs

142 DEPOSITIONAL CARBONATE RESERVOIRS

5.5. Assuming an average porosity of 12% and S w = 25%, what is a reasonable
estimate of OOIP for the Oaks Field? What is the average value of each of
the 13 wells with $ 75 oil? Assuming a drilling cost of $ 95/foot for this area in
1979, what did it cost to reach the Smackover reservoir at 3.5 - km depth? What
errors might result in a geological cross section based on wireline logs from
three wells, each in the center of the barriers and none on the fl anks?
5.6. Tidal fl at – lagoon successions may produce from diagenetically altered poros-
ity, particularly in the transition zone between supratidal and upper intertidal
zones, where fenestral porosity, intraclastic conglomerates, and scattered
grainstone – packstone beds occur. One reservoir example is Cabin Creek
Field, Williston Basin, Montana (Roehl and Choquette, 1985 ). This Silurian -

aged reservoir is about 3 km × 14 km in total area, of which 8100 acres are
productive. Average porosity and permeability are 15% and 5 md, respectively.
Assume one pay zone about 12 feet thick, with S w = 35%. Forty - fi ve percent

of the porosity is fenestral with pore sizes ranging from 0.5 mm to 1 cm in
diameter. The remaining 55% of porosity consists of sheet - like pores that
connect most of the fenestrae, along with intergranular pores in the fi ne sand -
sized peloidal matrix. The main rock types are wackestones with interbedded
flat - pebble conglomerates and peloidal packstones, algal - laminated mud-

stones, and evaporites.
(a) What is the orientation of the reservoir body with respect to depositional

dip and strike?
(b) What is the estimated OOIP?

(c) During field development, what potential problems may be encountered
because of facies variability?
5.7. What part of the shallow subtidal zone should offer the greatest potential for

depositional reservoir porosity? Why?

5.8. What is the anatomy and orientation of typical slope - break grainstone succes-
sions? These grainstone complexes consist of overlapping lime sand lobes
shaped by different episodes of vigorous tidal currents.

(a) What potential problems could this anatomical variability pose for geolo-
gists attempting to map the size and shape of fl ow units?

(b) Referring to Chapter 2 , what kinds of reef characteristics might appear in
slope - break buildups of Mesozoic, Cenozoic, and Paleozoic age?
(c) What classification scheme can be used to help identify the degree and

kind of variability in reef porosity?

(d) What information in the Lucia classification of carbonate porosity might

indicate the need for caution in calculating recoverable reserves in reef
reservoirs?

5.9. What exploration strategies would you use to find rocks with the greatest

depositional porosity in slope - toe deposits?
(a) What distinguishes between turbidites or grain flows and ordinary basinal

bottom sediments?

(b) What types of slope - toe deposits probably have the greatest depositional
porosity and permeability?