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ABSTRACT

The gently dipping (3-5° SW), complexly faulted, homoclinal structure of the Todhunters Lake Gas Field is deeply incised by a southwest trending tributary of the Markley Submarine Canyon. Formed during late Eocene, the canyon eroded Eocene Nortonville Shale through upper Cretaceous Starkey sands. Hemipelagic sediments infilled the canyon during early Oligocene creating an impervious barrier to hydrocarbon migration.

Faulting is an equally important trapping mechanism, particularly in sediments not affected by the incisement of the Markley Canyon, i.e., lower Starkey and Winters sands. A series of subparallel, NW trending, normal faults traverse the field creating isolated dry gas reservoirs. With offsets usually less than 80 feet, faults are generally not recognizable on available seismic lines. Some controversy exists over the age of faulting, but subsurface data suggests that it predates canyon incisement. No discontinuities are seen in the structure of the base of the Markley Canyon fill and well data in the canyon indicates no offset.

Since the discovery of the field in 1967, gas has been successfuly produced from the upper Cretaceous First, Second and Third Massive sands of the Mokelumne River Formation as well as the Starkey One through Five sands. Significant production has also been obtained in the upper Cretaceous Winters sands in the western half of the field. Cumulative production to 1982 is estimated by the Division of Oil and Gas to be 79,761 MMCF. Future development of the field will depend largely upon the identification of subtle traps controlled by faulting.

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