Introduction: from time to time we are , at Al-Bawaba are very proud to bring you , small taste of “old-times” hardships. Here is a feature that is brought to you by the kind permission of DGSONLINE.ORG, which deals in those early days.
Nestor John Sander, ”Sandy”, is now eighty-five years old. He is the last geologist living who worked for Aramco before World War II. He wrote the report recommending the Abqaiq structure as the site for a wildcat.
Later he reported the reversal of plunge on the En Nala anticline (now Ghawar). After the war, he was given responsibility for overseeing geology on wildcats. He left in January 1952 to set up a lab in New York.
In the leave intervening he studied for a D.Sc. at the Paris University. Sandy has just finished a book about Ibn Saud.
We would recommend you to visit Sandy’s well documented web page sat: http://home.inreach.com/rotsen/
N.J.Sander explains to us the importance of structure drills in the early oil exploration of the Eastern Province, and why this methodology proved to be efficient.
Historical context :
In the five years after the signing of theconcession agreement between Standard Oil of California (now Chevron) and KingAbd al-Aziz Al Saud in May, 1933, field work by Koch, Brown, Steineke, Hooverand Henry had progressed to the point that a generally reliable overview of thesedimentary sequence on the foreland of the Arabian shield was to hand, andprecise details were being added by a group of younger geologists under Max Steineke, now Chief Geologist.
Regional mapping using reflection seismic methodswas under way, and a gravity survey was planned. Second order triangulation hadset up a net of accurately located points marked by metal flags set in concreteand that work was ongoing.
Max had as an assistant Dick Bramkamp, a paleontologist with a Ph.D. from UC Berkeley who hadjoined him in 1936.
In June 1938, Dammam Well No.7 proved the commerciality of Dammam Dome, and on December 9th or 10thof that year reinforcements arrived to speed exploration. I was one of them.
Before my arrival seven shallow wells had traversed a large portion of the Paleogene to include the upper limit of strata underlying anhydrite tentatively dated as lower Eocene.
A wildcat, El Alat No.1, had encountered a Paleogene and older sequence much thicker than its outcrop on Dammam Dome where no anhydrite is present.
In many localities seismic work had encountered difficulty in obtaining records and interpreting them because the shallow stratum of anhydrite was not continuous.
It had either been leached awayor was replaced by another lithology. These shallow wells had shown that the other rock units of the Paleogene (upper Eocene and Oligocene absent) were persistent in lithology and fauna over extensive tracts measured in at least tens of miles.
A question remained: did the attitude of these young strata conform to any degree with that of producing levels ? Obviously, uplift at Dammam dome had continued into the Tertiary, but what about the enormous tracts where salt had not up welled ? Max had seen and reported some large features where topography and mappable beds suggested the existence of structure, but the mappable strata were Miocene or younger laid down un conformably on a Middle Eocene surface, eroded to an unknown extent.
Under Dick Bramkamp my first job was to log the sequence of and to map the Eocene over one of these features known as Abqaiq (Abu Qaiq, Father of Sand Flies). Much of the area was covered by sand dunes. The first well, S-8, was sited in the sabkha of Half MoonBay, a short trip from Dhahran, so the driller, Ray Winger, and I commuted to the site daily.
Structure drill rig used in 1952:
After consultationwith Max, I sited S-9 on what he thought might be the axis of the north-southtrending anticlinal feature.
The well was in a clearing bounded by low sand dunes. I used the plane table to run in the elevation from aflag, and found that the markers, both lithologic and faunal, were severalhundred feet higher than in the sabkha at sea level.
They were also closertogether. Access was difficult because of dunes, so we finally set up a tentcamp, with cook, mechanic, and soldiers. The next well S-10 was in a valley sometwenty kilometers due west of S-9. Max and I were both pleased to find the rockunits thicker than in S-9, and lower in elevation.
The next well, S-11,was the critical one, sited some twenty kilometers south of S-10 in the samevalley. If it found Eocene markers low structurally, a closed feature in strataof Eocene age could be postulated because regional dip was to the east.
They were low, so I completed the survey by sitting S-12 on what I thought to be thehighest elevation on the feature. I could see clearly a slight downward slope to S-11 from there.
To ice the cake I sited S-13 to S-16 in the quadrants around S-12 and found all four to be lower than S-12. So I wrote my report, made a mapand submitted them to management in the early summer of 1939.
My map showedlimiting closure of 800 feet (it is more than 3000’ at theArab-D). The wildcat wasspudded in February 1940 and I was chosen to “sit” it. The location was several hundred meters from S-12 because of topographic considerations.
I didn’t takethe well all the way, but it was completed as a producer and four step-outs werebrought in before my departure in June 1941.
It proved to management that enormous reserves existed in Saudi Arabia. I was charged with calculating the ‘proven’ reserve while the first four wells were completed, and although I was extremely conservative, the total mounted rapidly.
The Ghawar discovery:
This successful demonstration that structure at producing levels was reflected by near-surface strata, albeit in much attenuated form, set off a flurry of activity using thesame techniques and tools.
During my final pre-war year there were three rigs working under my supervision on the En Nala anticline, now the Ghawar structure.
We mapped the feature south 175 miles with no reversal on a gently-rising north-south trending axis with holes spaced at twenty to thirty kilometer intervals both east-west and north-south.
Only on the night of the Italian bombing did I report in code that S-108 (near Ain Haradh) showed a south ward plunge and drove, fortunately with a full moon, with blued-out headlights to Dhahran.
After the war structure drilling continued with three parties in the field and one offshore. The camps were provided with large trailers arranged as laboratories, with sinks and electric power.
Kitchen trailers furnished with ranges using bottled gas were standard. However,air-conditioned sleeping trailers were not yet. Rows of sleeping tents, one per person, were still used.
Why structural drill works?
In eastern Saudi Arabia everything is favorable to the use of the structure drill to define shallow structure: low regional dip, a broad epicontinental sea and apre ponderance of chemical sedimentation during a period of orogenic tranquility.
All contributed to a regional uniformity and continuity in rock units and theirfauna. The plate of fossils shows the three we used as markers. The occurrence of Lockhartia tipperi in the uppermost Umm er Radhuma formation signaled the cessation of drilling.
In the earliest days Alveolinasubpyrenaica, located above the anhydrite was the terminator, but the variationin the thickness and occurrence of anhydrite led to erroneous mapping, so the target was moved downward.
The extent to which the structure drill penetrated the whole Eocene-Paleocene sequence is shown bythe chart from my thesis that lists the remarkable benthonicfauna above the Cretaceous-Tertiary contact. Unfortunately the fauna of the whole of the Paleogene sequence vanishes northward as the sea deepened.
The most remarkable fact insuring the success of the structure drill in mapping older structure is that basement horsts continued to rise slowly but continuously during Mesozoicand Cenozoic times.
The gravity map by Thrallset al. in the AAPG bulletin (V. 43, N. 2,1959) shows basement highs clearly. The effects of this persistent movement are reflected not only in the Tertiary but also in older stratigraphic elements: the Arab-D member thins over the Abqaiq structure and younger pre-erosion strata exist down flank at the unconformity between the Wasia and Aruma formations, ahiatus that owes its existence mainly to epeirogenesis.
Whether or notCambrian salt is involved everywhere in this mechanism of continual upwardmovement is debatable. But regardless of mechanisms, the fact that olderstructure is reflected in younger rocks, albeit with great attenuation, led tothe discovery of a number of oil fields using a procedure unrelated to currentmethods of exploration.
Nestor J.SANDER
Source: (dgsonline)
© 2001 Mena Report (www.menareport.com)
