A Fan Tale, Modern and Ancient Fans--A Comparison

FISCHER, PETER J., California State University, Northridge, CA, VICTOR B. CHERVEN, Consultant, Shingle Springs, CA, and DEVIN R. THOR, Tetra Tech, Inc., Pasadena, CA

The Quaternary Conception fan of the Santa Barbara basin and the Upper Cretaceous Lathrop fan of the northern San Joaquin basin tell an interesting tale. Both fans show a well defined sequence stratigraphy of alternating low-stand, sand-rich units that alternate with thin high-stand silt units that drape and in-fill the surface topography of the previous sand-cycle. Isopachs made from detailed well log correlations (Lathrop) and seismic reflection data tied to borings (Conception) show that the fans are composed of a series of offset-stacked, elongate fan lobes. These lobes are similar in size.

Conception:

width of 2-10 km x length of 11-23 km x thickness of 30-50 m

Lathrop:

width of 6+ km x length of 15-20(?) km x thickness of 180 m

A major difference in the development of the two fans is the timing of tectonism. Concomitant tectonism uplifted the Conception fan lobes and resulted in localized erosion of high-stand silts beds and sand-on-sand lobe contacts. Tectonism at Lathrop occurred after fan deposition and provided the trapping structure--the Lathrop anticlinal fold.

Following are some lessons to be learned from these and other fans we have studied.

Quaternary or "modern" fans and ancient fans are similar.

Elongate sand-rich fan lobes separated by highstand silt units are typical of fans.

In addition to well-known techniques (seismic stratigraphy and detailed well log correlations), original reservoir pressures may be used to differentiate sequences and lobes (e.g., Lathrop).

Tectonism and erosion along the margin may limit traps to the uppermost lobe sequence (e.g., Conception).

An offset-stacked elongate fan lobe model is a valuable exploration and production tool.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)