Outcrop Expression of Confined Channel Complexes

Campion, K.M., A. R. Sprague, ExxonMobil Production Research Company, Houston, TX 77252-2189; M. D. Sullivan, ExxonMobil Exploration Company, Houston, TX; D. C. Mohrig, ExxonMobil Production Research Company, Houston, TX 77252-2189; J. A. Ardill, ExxonMobil Exploration Company, Houston, TX, R. W. Lovell, ExxonMobil Production Research Company, Houston, TX 77252-2189; P. A. Drzewiecki, G. N. Jensen, and D. K. Sickafoose ExxonMobil Exploration Company, Houston, TX

Abstract

Analog data for many seismically resolved confined-channel complexes comes from outcrop studies that provide insight into the lithofacies distribution and architectural features at the bed, bed set, channel, and channel complex scale. The Capistrano Formation (Miocene) near San Clemente, CA, is a sand-dominated system that is about 20 m thick and 1.3 km wide and serves as a model for confined channels that are represented seismically by a single cycle. In contrast, formations exposed in northern Californi a, such as the Bidwell Point and Gravelly Ridge, are gravel-dominated systems, 60-200 m thick, 500 m to 2 km wide, and serve as a model for confined channels represented seismically by multicycles.

The Capistrano is made up of laterally amalgamated channels that exhibit a change in net to gross ratio, facies preservation, and bed architecture from the channel margin to axis. Channel-margin facies is low net to gross (<50% net), non-amalgamated, thin-bedded, and dominated by a low-concentration of turbidites. In contrast, the channel-axis facies is high net to gross (>90%), thick bedded, amalgamated, and dominated by a high-concentration of sandy turbidites and gravel-rich tract ion deposits. The channel-margin facies is 200-300 m wide, whereas the channel-axis facies is 600-700 m wide. The Bidwell Point is made up of two sequences, which are marked by incision into slope mudstone, and are followed by deposition of gravelly rocks in the channel axis and both low and high-concentration turbidites on the channel margin. The channel-axis facies in both sequences consists of amalgamated channels dominated with granule and pebble conglomerate that exhibit traction features such as pebble imbrication, inclined beds, and alignment of flat pebbles. The primary channel margin facies consists of n on-amalgamated high- and low-concentration turbidites and local mud-draped erosion surfaces. The lower sequence consists of two stacked channel complexes up to 100 m thick. The base of each complex is marked by a muddy debrite. The lower complex is about 700 m wide and is primarily an axial facies, whereas the upper channel complex is about 2 km wide and has an axial facies that is 600 m wide. The top of both the lower and upper sequence is marked by an abandonment phase consisting of thin-bedded, silt-d ominated turbidites.

The Gravelly Ridge section studied consists of at least 5 stacked channel complexes separated by mudstone intervals. These channel complexes are 500 m to 1 km wide, 60-100 m thick, and extend 12 km in a dip direction; no distinct down-slope fining has been detected. Based on outcrop expression and paleocurrent patterns, these channel complexes are interpreted as sinuous systems dominated by an axial facies consisting of amalgamated granule and pebble conglomerate that display tracti onal features such as pebble imbrication, flat-pebble alignment, and inclined beds.

From a reservoir standpoint, the channel-axis facies in all the studied outcrops has the best potential for lateral and vertical bed continuity. The channel-margin facies with non-amalgamated beds and mud-draped erosion surfaces has low vertical and lateral continuity.