Architecture of Winters Submarine Fan, Sacramento Basin, California

Victor B. Cherven

A hierarchy of submarine-fan architectural elements and stratigraphic discontinuities, repeated through 2,000 ft of strata, gives the Winters formation a complex stratigraphic architecture that is the primary control on gas distribution in 18 of 22 fields. The formation has been mapped locally (in detail throughout individual gas fields), areally (in moderate detail over two or more townships), and regionally (throughout the basin) in order to examine reservoir continuity at various scales and delineate favorable subenvironments for stratigraphic traps.

First and second-order elements and discontinuities, visible in cores, include cross-bedding sets and set boundaries deposited in sandy bed forms under steady flow conditions (first order) and Bouma sequences deposited under varying flow conditions (second order). Third-order elements include individual sediment gravity flow beds and overlying thin beds or partings of hemipelagic mudstone that create permeability discontinuities and give a serrate character to log motifs. Cross sections indicate that most flow beds are local in extent and have relatively conformable bases, but erosion surfaces and amalgamated beds are evident visually or in core x-radiographs.

Flow deposits occur in fourth-order channels and mounds, forming areally more extensive sandstone packets that are bounded by thicker, more continuous mudstone intervals above and erosional, locally sharply discordant bases below. Fan lobes are still-larger fifth-order bodies that comprise groups of packets and are bounded by mudstone blankets that form distinctive low-resistivity regional marker beds. Lobes onlap adjacent lobes and offlap underlying lobes, forming undulating sandstone sheets (members) separated by mudstone. One of the mudstone blankets separates the stacked sheets into two major sequences representing two distinct fans.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.