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. Within the Capistrano Formation (Miocene) near San Clemente, California, is a sand-dominated complex of channel fills that is about 15 m thick and 1.2 km wide and serves as a model for seismically defined, single-cycle reservoirs. In contrast, channel fills exposed in northern California, such as the Bidwell Point (Valanginian) and Gravelly Ridge lenses (Tithonian) of the Stony Creek Formation, are gravel-dominated, 60–200 m thick, and 500 m to 2 km wide and serve as analogs for confined channels defined seismically by multiple cycles.

The Capistrano example is made up of laterally amalgamated channels that exhibit a systematic change in sand fraction, facies preservation, and bed architecture from the margin to axis of each channel fill. The channel-margin facies is low net to gross (<50% sand), non-amalgamated, thin-bedded, and dominated with low-concentration turbidites. In contrast, the channel-axis facies is high net to gross (>90% sand), thick bedded, amalgamated, and dominated with high-concentration sandy turbidites and gravel-rich traction deposits. In the Capistrano case, the channel-margin facies is 200–300 m wide, whereas the channel-axis facies is 600–700 m wide.

The Bidwell Point example consists of two sequences that are marked by incision into slope mudstone followed by deposition of gravel in the channel axes and both low and high-concentration turbidites on the channel margins. The channel-axis facies in both sequences consists of amalgamated channels dominated by granule and pebble conglomerates exhibiting traction features such as clast imbrication and inclined beds. The primary channel margin facies consists of non-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. Locally, muddy debrites mark the base of channel complexes. The lower complex is about 700 m wide and is primarily axial facies, whereas the upper channel complex is about 2 km wide and has an axial facies that is 600 m wide. An abandonment phase consisting of thin-bedded, silt- dominated turbidites marks the top of both lower and upper channel complexes.

The studied section of the Gravelly Ridge lens consists of at least 5 stacked channel complexes separated by mudstone intervals. This set of channel complexes is 500 m to 1.6 km wide, 60–100 m thick, and extend at least 12 km in a down-dip direction; no distinct down-dip fining was detected. Based on outcrop expression and paleocurrent patterns, these deposits are interpreted as filling sinuous channels and primarily are amalgamated granule and pebble conglomerate displaying tractional features.

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