Old field gets new image

Deep Logging Key to Understanding

The collective creative minds that abound in the vital oil and gas industry excel at overcoming the myriad challenges indigenous to this risky, high stakes business.

They're particularly adept at figuring out how to squeeze more hydrocarbons out of tired old fields.

A noteworthy example is the 1,200-acre Inglewood oil field along the Newport-Inglewood Fault trend in California's Los Angeles Basin. The field was discovered by Standard Oil of California (Chevron of today) in 1924.

Initial development at Inglewood was based mostly on drilling topographic highs and establishing production from the shallow Upper and Lower Pliocene zones, where the geologic structure was better understood.

Oil and gas production at the field was on the decline when Stocker Resources – a predecessor to current field operator Plains Exploration & Production Co. (PXP) – stepped in and acquired Inglewood in 1990 and began applying its expertise to ramp up production rates.

Since then the focus at Inglewood has centered on understanding the detail reservoirs, starting from the top and working down to the Sentous unit, according to AAPG member Dalton Lockman, senior geologist at PXP.

The Sentous occurs beneath the Moynier, Bradna and Nodular Shale units; the Nodular Shale has been a key horizon in mapping the subsurface.

PXP revved up the action in the field in 2003 when it embarked on a development program targeting older, Middle Miocene zones. Prior to this time the field had produced 350 million barrels of oil from the original shallower target intervals.

"Results from recently drilled wells indicate that geologic structure is a dominant factor in accurately understanding and identifying deeper zones with new reserves in the field," Lockman said. "These lower zones are structurally complex reservoirs that present challenges both in characterization and subsequent development."

The Goal is Met

In addition to conventional triple combo open hole logging suites used on 30 of the 40 development wells drilled to an average depth of 8,300 feet into the lowermost zones, it is noteworthy that electrical micro-image borehole logs also were acquired. 

"We shot seismic over the field, and once we said ‘here's our play' and started to drill we put together the formation evaluation program with image logs right at the front of that," Lockman said. "We needed to understand the geometries of the geologic structure, and that was the premise of running image logs up front."

The program employed Halliburton's X-tended Range Micro Imager (XRMI™) tool designed for fresh mud, as well as its predecessor – the EMI™.

Information provided by the borehole image logs acquired at Inglewood has been used to:

  • Resolve thrust fault geometries in the Miocene.
  • Interpret and re-map the subsurface below the Nodular Shale unit.
  • Identify igneous units not distinguishable on conventional logs.
  • Provide fracture characterization for use in successfully completing within igneous units.
  • Refine the selections of intervals for perforation and stimulation within intervals of sand.

"The image logs gave us a way to evaluate the deep structure," Lockman noted. "There were over 1,300 traditional well logs in the field from wells dating back to discovery, with 50 or so to the Sentous reservoir, the target of our deep drilling program.

"The issues were being able to map thrust features at those depths, because there's a lot of high angle beds with thickening and thinning shales," Lockman said. "The image logs let us take a look at the structure and interpret it differently than before.

"Going in, that was our main goal," he noted. "We were drilling in and around the Newport-Inglewood fault, a strike-slip feature that had been mapped down to the basement with complex folding on both sides of the fault."

A Deeper Understanding

The image logs, the company said, also proved invaluable to determine the best way to complete the wells and to look for re-completion opportunities.

As an example, Lockman cited a re-completion effort that took place in the last year in the Sentous zone:

"There are some igneous rocks, volcanic facies there that we were able to identify on the image logs, and we also identified they were highly fractured," he said. "We re-completed some of those wells in volcanics, which was not pay zone for us initially."

The wells drilled in Pxp' Inglewood deep development program underwent frac completions, which entailed shooting in perfs and fracing with a frac sand behind that.

"To pick these perfs we also used the image logs to pick intervals we felt would take the perfs and take the fracs the best and give us the biggest advantage," Lockman said. "We were sort of optimizing the perfs, so to speak."

Lockman succinctly summarized the major role image logging played in successfully dealing with the geology of the deeper, Lower Pliocene and Upper and Middle Miocene strata of the Inglewood field.

"Conventional triple combo logs and traditional dipmeters hadn't been able to decipher this complex geology," he noted. "Image logging contributed with its capability to identify structural details, such as small folds, overturned beds, fractured intervals, thrust-faulted intervals, igneous rocks and thinly laminated reservoir rocks.

"The confidence developed through the use of image logging led to a new interpretation for faulting of the Inglewood Field at depth," Lockman said.

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Emphasis: Downhole Geology