California Kreyenhagen, deeper exploration advance

By OGJ editors

Zodiac Exploration Inc., Calgary, has established in its 1-10 well in Kings County, Calif., the presence of several deeper targets that its technical team had identified on 3D seismic shot in 2009 and is preparing to plug back and drill a horizontal leg into the Upper Kreyenhagen formation.

Zodiac plans to further evaluate the deeper zones with identified hydrocarbon potential in later wells in the northern San Joaquin basin. It drilled the 1-10 well, second evaluation well on its Jaguar prospect, to a total depth of 16,950 ft in Cretaceous.

The company also reported encouraging oil test results from the Whepley formation at the 4-9 well, its first well on the prospect. It also plans to seek a joint venture partner to advance the project to commercial status.

The company further deepened the 1-10 well, originally targeted to a true vertical depth of 15,240 ft, to test more zones of interest including the Cretaceous Moreno formation, a major source rock in the area, according to the US Geological Survey. Petrophysical log and core analysis confirm prospective targets in the Whepley, Vaqueros, Upper Kreyenhagen, and Lower Kreyenhagen formations.

The Upper Kreyenhagen zone, tested in the company’s 4-9 well, was cored in the 1-10 well. The overlying source shales and the siltstone reservoir target were cored with a total 110 ft recovered of a total Kreyenhagen thickness over 800 ft. Visual examination of the core confirms the presence of oil stained fractures.

The well reached total depth at 16,950 ft on August 28, 2011, after intersecting sandstone, siltstone, and shale sections below the Kreyenhagen formation with high background gas readings and associated increases in C1-C5 on mud logs, and oil shows.

With formation pressures estimated in excess of 14,000 psi and significant mud log hydrocarbon response in the lower 250 ft of the wellbore, mud weight was increased to 16.4 ppg before drilling was halted. As a result of deepening the well the Company has now identified several deeper potential targets.

Meanwhile, following a 45-ton fracture stimulation over a 30 ft interval in the Lower Vaqueros formation, the 4-9 well flowed water at a stabilized 57 b/d for 19 days. The well was then shut in for a buildup test. The pressure test analysis indicated a successful fracture treatment with a negative skin.

Before moving uphole, a total of 2,400 bbl of fluid was recovered from the zone. The reservoir is overpressured with 10,980 psi reservoir pressure and a gradient of 0.8 psi/ft. Zodiac continues to investigate several possible sources for the water.

The Whepley formation was later tested following a 67-ton fracture stimulation over a 30-ft interval. This zone flowed 33° gravity oil at a stabilized 13 b/d at a 40% pressure drawdown for 9 days before being shut in for a buildup test.

The pressure test analysis indicated significant skin damage of +7, which the company believes can be addressed by optimizing stimulation procedures in future wells. The forecast derived from the well test analysis demonstrated that a rate of 30 b/d could potentially be achieved with a pressure drawdown of 40% and the removal of formation damage from +7 to 0.

The Whepley reservoir is overpressured with 10,900 psi reservoir pressure and a gradient of 0.8 psi/ft.

The company termed the Whepley results encouraging because the perforations were placed primarily over the Lower Whepley shale interval due to wellbore constraints. The Whepley siltstone, a gross 50 ft thick, the primary reservoir in the Whepley formation, remains largely untested in this wellbore.

Petrophysical and core analysis indicate the presence of free oil in both fractures and matrix in this zone. While the occurrence of water in the Lower Vaqueros test is unexpected, this test does not negate the Vaqueros formation as a target.

Given the proximity of the 4-9 wellbore to the 1-9 wellbore drilled in the 1970s in which thousands of barrels of water were injected during well control operations, and the fractured nature of the rock, there is more than one possible source for the water, Zodiac said. Secondly, the entire Vaqueros formation is more than 650 ft thick. The company’s first test covered 30 ft of the lowermost 200 ft of the Vaqueros formation.

Oil-bearing sandstones above the tested zone have been identified in core and on well logs in both the 4-9 and 1-10 wells, but downhole equipment limitations did not allow testing of these zones in the 4-9 well. Further testing will be required to fully evaluate this formation.

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