CONTINUOUS-TYPE ACCUMULATIONS

                    
SANDSTONES, SHALES, AND CHALKS


     Continuous-type accumulations may have spatial dimensions 
approaching those of plays and cannot be represented in terms of discrete,
countable entities delineated by downdip hydrocarbon/water contacts, as
are conventional accumulations. The identification of a continuous-type
hydrocarbon accumulation is based on its geologic setting and does not
incorporate somewhat ephemeral criteria, such as specified low API
gravity2, low permeability ("tight"), special regulatory status, or
need for unusual engineering techniques. A low-permeability reservoir
may or may not be a continuous-type accumulation.

     The geologic setting typical of continuous-type accumulations is
illustrated by figure 5.  Common geologic 
characteristics of acontinuous-type accumulation include occurrence downdip 
from water-saturated rocks, lack of obvious trap and seal, crosscutting of
lithologic boundaries, large areal extent, relatively low matrix permeability, 
abnormal pressure (high or low), and close association with source rocks. 
Aspects of hydrocarbon production common to a continuous-type accumulation 
include a large in-place hydrocarbon volume, a low recovery factor, and a 
heterogeneous "hit-or-miss" character for production rates and ultimate 
recoveries of wells.

2 A standard adopted by the American Petroleum Institute for expressing
the specific weight of oils.

     In the case of continuous-type accumulations, the distinction between
undiscovered resources and inferred reserves is blurred. The locations
of continuous-type accumulations are commonly well known (implying inferred 
reserves) (fig. 6), but hydrocarbon estimates may be broadly
dependent on geologic knowledge and theory (implying undiscovered resources).

     The first step of the assessment procedure is to represent the
continuous-type accumulation by a play or plays. As in the case of
conventional accumulations, geologic risk is assigned to each play. A
gas-to-oil ratio of 20,000 cubic feet of gas per barrel of oil
separates gas plays from oil plays.

     It is advantageous to envision the hydrocarbons of a continuous-type
accumulation as residing areally in cells. A play is then regarded as a
collection of cells (fig. 7). The cell area or size is equal to the 
median spacing, as dictated by drainage area, expected for wells of the
play. Virtually all cells in a continuous-type accumulation are capable
of producing some hydrocarbons. For purposes of this assessment, however, a 
productive cell is one for which production from the play is formally reported.
An untested cell is one in which the play in question has not been evaluated by
a well.

     The second step of the assessment procedure is to estimate the number
of untested cells in a play and the fraction of untested cells expected
to become productive (success ratio). Realistic consideration of the
uncertainties associated with the number of untested cells in a play
usually leads to a substantial range between the minimum and maximum
number of untested cells. Therefore, the number of untested cells is
treated as a probability distribution.

     The third step of the assessment procedure is to establish a
probability distribution for estimated ultimate recovery (EUR) for
untested cells of the play that are expected to become productive. This
distribution provides a reference model for production from cells yet
to be drilled. Of course, this statistical model provides no insight as
to which untested cells are expected to become productive.

     Finally, the combination of play probability, success ratio, number
of untested cells, and EUR probability distribution yields the potential
additions to reserves expected for the continuous-type play. The in-
place hydrocarbon volume is not used in this assessment procedure.

     A salient aspect of the assessment method is that production and
development patterns of the past are projected into the future. No
assumptions regarding technology or economics are incorporated into the
model.

     The information required for the assessment of continuous-type
accumulations is supplied by earth scientists who are knowledgeable
about the petroleum geology and engineering of the province under
consideration. These regional experts complete a data form for each
play, which is the source of the input data required for assessment-
computation programs. In those few cases in which there were seriously
discordant views regarding a continuous-type play, the opinion of the
province geologist has been used.

     In order to aggregate continuous-type accumulations, a slight
modification of the procedure used for conventional accumulations was
employed: (1) the three basic attributes of geologic dependency of the
assessment units considered were charge, reservoir, and areal extent
(note that the dependency associated with areal extent could be
negative), (2) correlations could assume any value between 1.0 and
-1.0, and (3) dependencies at all aggregation levels were estimated in
the same manner as that described for conventional plays.