Abstract: High-Resolution Correlation of Coastal Plain Strata for Definition of Reservoir Attributes
Jean Christophe Navarre, Timothy A. Cross
Three types of stratigraphic cycles are recognized in coastal-plain strata of the Mesa Verde Group (Cretaceous), San Juan basin, Colorado. The cycle types change as a function of accommodation and geographic position. The cycles record oscillations of base level and concomitant oscillations of increasing and decreasing accommodation. Coastal-plain strata were traced physically into shoreface strata to establish temporal equivalency of strata within the two environments, and thus ensure correct recognition of baselevel rise and fall hemicycles in coastal-plain strata. The stratigraphic cycles establish a high-resolution correlation framework and a strategy for correlation in continental environments. Potential reservoir units are channelbelt sandstones and backbarrier and ay sandstones which occur in two of the cycle types. These sandstones change geometry, connectivity, and volumes within cycles, reflecting changes in accommodation, the types of original geomorphic elements within environments, and sediment preservation during base-level cycles. The occurrence of different facies successions in the different types of cycles establishes the empirical basis for recognizing such cycles with well-log inversion techniques.
One type of stratigraphic cycle, which occurs in the lowest accommodation condition of the Mesa Verde Group, is an alternation between laterally amalgamated channels and either crevasse splay/crevasse channel complexes or floodplain vertical accretion deposits and soils. Laterally amalgamated channel sandstones accumulated during base-level rise hemicycles. Vertical facies successions within these sandstones record increased preservation of bedforms and, therefore, increasing accommodation from a minimum at the basal, low-relief erosional unconformity. Crevasse splay complexes accumulated during baselevel fall. Facies successions within crevasse splay channel sandstones record decreased preservation of bedforms, and therefore, decreasing accommodation from a maximum at the base of the prograding crevasse splay complex.
A second type of stratigraphic cycle, which occurs in the maximum accommodation conditions of the Mesa Verde Group, does not change facies associations alternately in base-level cycles. Instead, these cycles are symmetrical, in the form of ABCDCBA. The base-level fall hemicycle consists of the following facies succession: gray-green blocky, lacustrine mudstone lacking concentrations of plant detritus; "paper shales" (fissile mudstones with dense concentrations of plant detritus on laminae surfaces) of lake origin; silty paper shales; and either coal, floodplain vertical accretion siltstones, crevasse splay siltstones, thin sheet sandstones or poorly developed soils. Facies successions in the base-level rise hemicycle are opposite.
The third type of stratigraphic cycle also occurs in the maximum accommodation conditions, but in a more seaward position than the previous type. Like the first, this type of stratigraphic cycle comprises an alternation of two types of facies associations. This type is an alternation between tidally dominated estuarine and bay sandstones and mudstones which accumulated during base-level rise, and shallowing-up and bed-thickening up hummocky cross stratified back-barrier and bay sandstones which accumulated during base-level fall.
AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France