And the Faults Hid Their Charms
Corpus Bay Fields Hid Their Faults
The use of 3-D seismic data has become so commonplace in the E&P world that for the most part it’s now considered just part of the everyday routine.
Yet 3-D applications continue to generate results akin to near-miracles in certain situations.
A notable example can be found in Corpus Christi Bay off the lower Texas coast, where Royal Exploration Company partnered with Sabco Oil & Gas Corp. in 1997 to exploit state leases in the bay.
The leases were held by production by Sabco via remnants of Frio production discovered years earlier.
“The partnership concentrated on two fields – East Corpus Christi and Encinal Channel – which are downthrown to a large growth fault, which extends across the mid-section of the bay,” said Robert Rice, geologist at Royal. “The two fields are located on a shale cored ridge, which is perpendicular to the growth fault.”
At the time, the structurally simple, non-geopressured East Corpus Christi field had produced 90 BCF from Upper Frio sands at maximum depths of 9,000 feet subsea.
In contrast, Encinal Channel, which becomes the dominant structural feature below this depth, produced more than 150 BCF from mostly geopressured Frio sands, where the faulting becomes increasingly complex with depth.
“Encinal Channel is largely unfaulted above the geopressured interval and was adequately exploited by 1960-era technology,” Rice said. “The key to additional reserves was the advent of complex faulting below the pressure point – about 9,500 feet subsea – combined with a stratigraphic section with thicker shale sections between the sands.
“This combination led to multiple high-side fault traps on the overall large Encinal Channel anticline.”
Get a Clue
The entire two-field complex had been discovered and exploited beginning in 1952, before high quality 2-D data were available.
It is noteworthy that Royal and Sabco struck their partnership deal the same day that Western Geco released 80 square miles of speculative 3-D data in western Corpus Christi Bay.
“When Western came in, the bay had produced 375 BCF – before 2-D was any good – from fields under the first 80 squares they shot,” Rice said. “It was just the perfect place for 3-D.”
A “clue well,” the Gulf Oil ST 48 #2, provided Royal and Sabco with the confidence to license the new data.
The 48 #2 well had encountered the geopressured Middle Frio Nodosaria blanpedi M-4 sand at 10,588 feet subsea. The M-4 was perforated within 10 feet of the gas-water contact and went to water after producing 2.6 BCF.
Subsurface control showed the 48 #2 well to be 335 feet low to the Cities Service ST 49 #1 discovery well for the field.
There appeared to be 700 acres of un-produced prospective territory between the two wells, where a third well encountered a barely productive M-4 sand, which was almost completely faulted out, according to Rice.
He noted the initial 3-D interpretation revealed several undeveloped fault blocks in the area.
Subsurface and 3-D mapping identified other potential targets besides the M-4 sand. These included the shallower M-2 sand – a known producer in the area – along with sands deeper than the M-4, ranging through the M-10.
The M-2 and M-4 sands worked as predicted in the partnership’s initial well at Encinal Channel, but the deeper sands were wet. The well was dually completed after a vertical seismic profile (VSP) was recorded, revealing that the mapped M-4 reflector was in the wrong position. The correct reflector had an amplitude signature, which led to extensive development of the M-4 sand.
Further review of the data, now mapping on the deeper M-4 reflector, revealed a small cross fault segmenting the reservoir and buried below the original mapping reflector, Rice noted. This indicated very small faults had the potential to trap the M-4 and to segment larger fault blocks.
The second well the partnership drilled in Encinal Channel was the Sabco 48 #7, where a couple of deeper sands were completed but failed to produce economically, while the M-4 sand was fully loaded, according to Rice.
He noted it has become the champion producer in the field, having now made approximately 13 BCF and 400,000 barrels of condensate.
The partnership’s focus shifted entirely to exploiting the multiple M-4 fault blocks after a third well once again failed to find deeper commercial production.
The result of this new focus was 17 successful wells and three dry holes, leading to peak production of 60 MMCFG/D, with the added production to date tallying approximately 40 BCF and 800,000 barrels of oil.
Early in 2007, the wells were still kicking out 20 MMCFG/D.
“The main thing was finding that M-4 sand and finding it much more faulted than we expected,” Rice said. “We found a whole lot of fault blocks you couldn’t see with subsurface mapping – and that’s really the story of this whole thing.
“An old Cities Service map constructed around 1967 shows what they thought it looked like when they quit drilling,” Rice said. “The map was so simple, and that’s all you could do is subsurface control.
“The other big thing was once we got on the right reflector after running the initial VSP, we found there was an amplitude anomaly and the fault blocks that basically had it were the best ones,” Rice noted. “We started off one leg too high, and when you’re on the wrong leg – amplitude or not – it’s wrong.”
Using What You Have
Rice emphasized they really didn’t find anything startling or new but were merely exploiting a field that was already there.
“To a degree, it’s the story of 3-D,” he said. “There are hundreds of these same things where people went in and shot old fields and found a bunch of reserves.
“But there’s nothing like a great ‘clue’ well to help initiate a project,” he added.
For a geologist, there’s also nothing quite like watching your well being drilled while enjoying the comfort of your own home.
“I’m three houses from the bay, so it’s in my front yard,” Rice said, “and at the (GCAGS) convention we’ll be looking out at the wells.
“Every Texas geologist should be able to see his drilling rig from the front yard.”