Understanding the Diversity of Lacustrine Hydrocarbon Reservoirs—The Utility of Outcrop Studies

Abstract

A wide variety of lacustrine carbonate strata can act as hydrocarbon reservoirs. Such carbonate geobodies have highly variable morphology, connectivity, and reservoir quality that are challenging to decipher from subsurface data alone. Incorporating insights from outcrop studies reveals that the reservoir potential of lacustrine carbonate geobodies is strongly influenced by lake-basin type, depositional subenvironment, and sequence-stratigraphic position. Study of outcrop analogs enhances understanding of reservoir distribution (heterogeneity, 3D distribution of flow units), depositional controls, well performance, reservoir modeling (geobody dimensions), and well-log and seismic expression. Outcrop exposures allow one to examine lateral and vertical variability at the parasequence scale, map systematic variations in geobodies at the scale of a subsurface field, and relate potential reservoirs to coeval baffles and barriers.

In addition, outcrop study make it possible to relate depositional morphology and other geomorphic characteristics of carbonate geobodies to syn- and post-depositional diagenetic changes. These changes appear to be a function of lake-basin type (LBT): pore fluids are affected by not only the general balance of sediment+water relative to potential accommodation, but also by the details of the source of water (meteoric, connate, deep hydrothermal) and its influx path (direct precipitation, overland, subsurface).

Finally, outcrop observations and interpretations can be placed readily into the larger-scale context of lacustrine systems and lake-basin types. This context is quite useful because each LBT has characteristic types and distributions of reservoir- and seal-prone strata as well as diagenetic alteration: Overfilled LBTs host fluvial, deltaic and shoreline reservoirs, including skeletal, algal, and coated grainstone with low Kv and average net:gross. Balanced-fill LBTs have both carbonate (microbial boundstone, skeletal and coated grainstone) and clastic reservoirs with good Kv and Kh. Underfilled LBTs host shoreline-carbonate and clastic sheetflood reservoirs with low Kv but thick net pay.

We illustrate these points with examples from the ExxonMobil Lacustrine Carbonate Collaborative study of modern and ancient outcrop analogs in 15 countries on 6 continents, of Cambrian to Holocene age, drawing mainly on the Cretaceous Yacorite and Apache Canyon fms, and Cenozoic Green River and Hot Spring fms.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018