Abstract: Application of Outcrop Studies to Aid Facies Interpretation, Establish High-Resolution Correlations, and Map Reservoir Compartments

Jean Christophe Navarre, Remi Eschard, Francois Guillocheau

As part of a project designed to use stratigraphically identified fluid-flow units as input for reservoir simulation, we have used surface correlations to aid subsurface correlations of strata within the Mesa Verde Group, San Juan basin, Colorado. Surface correlations show changes in symmetry of stratigraphic cycles and changes in facies types and associations within different parts of stratigraphic cycles. A knowledge of these changes in facies within a stratigraphic context is an invaluable aid in high-resolution subsurface correlation and in mapping fluid-flow compartments.

In miles-long continuous exposures in Mancos Canyon, we have identified very low-angle (0.12 degrees) clinoforms in shoreface strata of the Point Lookout Formation. These clinoforms extend continuously into more horizontal coastal plain strata of the Menefee Formation. Strata within groups of clinoforms of base-level fall hemicycles show shallowing-up vertical profiles in shelf through upper shoreface facies. The shallowest facies of the most seaward (youngest) shoreface clinoform package is overlain either by a thin deepening-up facies succession or by a transgressive surface of starvation followed by another shallowing-upward facies succession.

As strata of base-level fall hemicycles are traced more landward, additional clinoforms bounding additional strata appear above the base-level fall clinoform packages. These belong to base-level rise hemicycles. Facies above and landward of the base-level fall upper shoreface are sandy and heterolithic tidal deposits. These vary from 1/4 to equal the thickness of the shoreface. Farther landward base-level hemicycles are recognized by alternations in types of channelbelt sandstones and lake and floodplain mudstones. Base-level rise strata generally show increasing aggradation and sediment preservation in a landward direction for a few miles.

The most continuous, and often the thickest, mud-rich strata occur at the base-level rise to fall turn-around positions in all environments. These continuous, low-permeability strata follow the topography of clinoforms and divide the reservoir quality sandstones of the shoreface, tidally influenced estuary and bay, and channelbelts into reservoir compartments. Sandstones of the compartments average 10-15 m thick, dip at 0.12 degrees through the shoreface, extend an average of 13 km in the dip direction, and at least 10 km in strike direction. Mud-rich compartment boundaries are up to a few meters thick and are coextensive with the sandstone compartments. The Point Lookout Formation comprises innumerable such compartments that vary in their ratios of aggradation to progradation.

These observations facilitate correlation of subsurface strata. They also help the interpreter understand why some shallowing-up profiles are sharp, why others are symmetrical, and why yet others are fining up, all within the same time-bounded stratigraphic package. A knowledge of these changes in facies is an invaluable aid in mapping fluid-flow compartments.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France