ABSTRACT Mount Diablo is flanked on its northeast side by a thick section of Late Cretaceous and Tertiary sedimentary rocks, which produced small hydrocarbon accumulations in the Los Medanos, Willow Pass, Mulligan Hill, and Concord gas fields. The first well was drilled in 1864, and today most of the active wells on the northeast flank are used for gas storage by Pacific Gas and Electric Company. These fields, which also include the Brentwood oil field, lie to the northeast of Mount Diablo and have produced 6.4 million cubic meters (225 billion cubic feet) of natural gas and over 57 million cubic meters (9.1 million barrels) of oil. The main reservoirs for the Sacramento Basin are sandstones in the Late Cretaceous and Paleogene section. The source rock there is primarily from the Upper Cretaceous Dobbins Shale, which began generation 75 m.y. ago, and the Winters Shale, which began generation 35 m.y. ago. The Livermore Basin is located on the western and southwestern sides of the mountain. The only commercial field in that basin is the small Livermore oil field. This field produces primarily from Miocene sandstones. The Livermore Basin is a Neogene basin that was syntectonically formed in the last few million years and continues to grow today. Studies of the black oils found in the Livermore field show that the source rock is likely the Eocene Nortonville Shale, though the Upper Cretaceous Moreno shale is also considered to be a possible source. The Livermore field has produced 12 million cubic meters of oil (1.9 million barrels).