Architecture of The Winters Submarine Fan, Sacramento Basin, California
Authors: Cherven Victor B. 
Published: November 2010

A hierarchy of submarine-fan architectural elements and stratigraphic discontinuities, repeated through 600 m of strata, gives the Winters Formation, Sacramento Basin, California, a complex stratigraphic architecture that is the primary control on gas distribution in most fields. The formation has been mapped in detail throughout individual gas fields, in moderate detail over two or more townships, and more generally throughout the basin in order to examine reservoir continuity at various scales and to delineate favorable subenvironments for stratigraphic traps. Discontinuities range from obvious small-scale erosional surfaces to large-scale, generally unrecognized mudstone blankets that separate the formation into discrete stratigraphic units.

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 sandstone beds deposited by sediment gravity flows. Cross sections indicate that most sandstone beds are local in extent and have relatively conformable bases, but amalgamation along minor erosional surfaces (third-order discontinuities) is evident visually or Sandstone beds are overlain by thin beds or partings of hemipelagic mudstone (third-order discontinuities) that reduce permeability and give a serrate character to log motifs.

Areally extensive bundles of sandstone beds form fourth-order channels and mounds. Stacked channels and mounds are separated by fourth-order discontinuities created by moderately extensive mudstone blankets that are several meters thick. Fan lobes are still larger fifth-order bodies that comprise stacked sequences of fourth-order features. Lobes are somewhat elongate in shape and extend from the lower slope to the lower fan. Lobes were fed by delta-front slope channels and are thickest in the upper-fan region off the mouths of these channels. During subsequent progradational events, abandoned lobes formed topographic highs on the seafloor that directed subsequent sediment gravity flows into adjacent lows; therefore younger lobes are offset-stacked onto the flanks of older lobes. The formation contains more than 25 of these aggradation/abandonment cycles (depositional sequences), and offset stacking of fan lobes gives these sequences a markedly undulating morphology.

Most lobes are bounded laterally by wedge-shaped bodies of mudstone that form fifth-order discontinuities in the reservoir. The mudstone blankets that overlie lobes are more extensive sixth-order discontinuities that form distinctive regional marker beds on resistivity logs. Fifth- and sixth-order discontinuities are not present, however, where coeval and laterally adjacent lobes coalesced or lobes scoured through underlying mudstone blankets into older lobes. Coalesced and amalgamated lobes form extensive sandstone sheets (sixth-order elements) that can be traced for tens of kilometers along depositional strike. Whereas isolated lobes that are encased in mudstone form discrete reservoirs that have separate gas-water contacts and different pressures, lobes within sandstone sheets appear to be in fluid communication.

The upper and lower members are the largest architectural elements within the Winters Formation. Each of these seventh-order elements is composed of several sandstone sheets and many fan lobes. The members are separated by a thick (50 to 100 m) mudstone blanket that can be traced ~100 km along strike.