Abstract

Cretaceous strata in the Sacramento Basin are characterized by lenticular sand bodies interbedded with more widespread mudstone, shale, and coal beds. Many of the sand bodies were deposited in topographically elevated channel-levee systems on delta plains and submarine fans, and the finer-grained facies were deposited in laterally adjacent overbank environments. During subsequent burial, differential compaction of the channel-levee and overbank-floodbasin facies accentuated the original depositional relief, forming antiformal structures over the sand bodies and, in some cases, normal faults along their margins.

Unlike tectonic anticlines, the antiforms die out rapidly with depth. They are seldom evident below the base of the sand body, but they may persist through several hundred feet of strata above it. They form mounded reflectors on seismic lines, but deeper reflectors are commonly flat and the basement surface may or may not be irregular, indicating that these structures are probably unrelated to basement anomalies.

Most sand bodies pinch out updip, as do the associated antiforms. Downdip, the sand bodies thicken and widen and antiforms broaden. Some sand bodies have undergone dip reversal due to compaction, forming doubly-plunging antiforms.

Non-tectonic antiforms are the trapping mechanism in several important gas fields in the basin. This paper illustrates examples from the Millar and River Island gas fields in the southern part of the basin.