?Northern California’s Sacramento Basin has been a notable gas-producing province for many decades, and more than 9 trillion cubic feet (Tcf) of gas has flowed from its fields. Accumulations are in an array of Tertiary and Cretaceous sandstones; some 16 horizons produce across the prolific basin. Although considered mature, the Sacramento’s production has been swelling from several of its largest fields, thanks to infill drilling and workover campaigns.


According to California’s Division of Oil, Gas and Geothermal Resources, production from Rio Vista, the state’s largest nonassociated-gas field, grew to 19.5 billion cubic feet (Bcf) in 2007, up strongly from 16.3 Bcf in 2006. Third-place Willows-Beehive Bend Field made 8 Bcf of gas in 2007, surging from 6.5 Bcf in 2006. And, Grimes, California’s second-largest nonassociated-gas field, kept production nearly flat, dropping just slightly to 10.8 Bcf in 2007 from 11 Bcf the prior year.


Field rejuvenation efforts are the reason. Such strategies are cornerstones of Venoco Inc.’s active Sacramento Basin program. The Denver-based company is the basin’s most active driller, and it is devoting $141 million—about half of its 2008 capital budget—to its assets there. It holds about 200,000 net acres, mainly in Grimes and Willows fields, and makes 55 million net cubic feet equivalent per day from the basin. That’s skyward from 10 million in 2004, when it began to build activity. Today, Venoco is responsible for 70% of drilling activity and 85% of workover activity in the basin. Last year, it spud 127 wells and worked over or recompleted another 113.


According to the company, the Sacramento Basin offers attractive and diverse opportunities. This year, it will drill more than 110 wells, frac 50, and work over more than 125. Its drilling projects are mainly 40- to 20-acre infill sites in the thousands of feet of gas-charged section present in its fields. It is also testing 10-acre pilots. Currently, Venoco has five drilling and five workover rigs engaged.


More potential lies in performing fracture treatments in existing wells. The old gas fields brim with sands that have never been stimulated—historically, wells were completed naturally in thick channel sands. That protocol was followed even when boreholes intersected thinner deposits such as overbank and levee sands. These small, stacked lenses, which typically extend across fewer than 20 acres, respond nicely to hydraulic fracturing.


Altogether, the company estimates it has more than 3,000 projects in inventory in the Sacramento Basin.


About half are workovers and recompletions, and the rest are infill locations and fracturing candidates. Potential net reserves range from 460 to 719 Bcf, and Venoco projects development costs between $1.38 and $2.53 per thousand cubic feet.


Houston-based Rosetta Resources is also active in the basin, where it makes 44 million net cubic feet equivalent per day from 179 producing wells. It plans to spend 19% of its $290-million 2008 capex budget in California.


Rosetta owns 76,000 net acres, mainly in Rio Vista, a 10-mile-long by eight-mile-wide massive gas field on a large structural dome. Rio Vista has already produced more than 3.5 Tcf of gas equivalent; its main reservoirs are Eocene Domengine sands, but gas occurs across a wide variety of additional pays. Last year, Rosetta drilled 27 wells and concentrated on infill drilling and extension of a play in the Paleocene Martinez sand.


This year, it plans 29 wells in California. Within Rio Vista, 20 wells will target field development and extension opportunities. It has two drilling rigs at work, and is keeping two to three completion rigs busy. And, it will work over between 30 and 40 field wells.


Additionally, Rosetta has been investigating possibilities in an extension area. It’s evaluating numerous structural and stratigraphic ideas, including low-contrast and low-resistivity pays it believes are underexploited. Nine wells are scheduled for the extension project.


At present, the company has more than 110 drillable locations in inventory, not including its considerable workover and recompletion opportunities.