It is one of the most remote places on the planet and home to the largest sand desert in the world. Virtually uninhabited and underexplored, the Rub’ al Khali basin, known as “the Empty Quarter” in Arabic, covers more than a fourth of the southeastern part of the Saudi Arabian peninsula.
Before high resolution satellite imagery, the tall red and yellow sand dunes that guarded unexpected oases were mostly known to the local Bedouin and desert guides. Before cars, travelers from the Orient managed to traverse the majestic yet challenging terrain by camel and recorded their journeys in published diaries.
While the area has been explored in the past, perhaps most notably by Max Steineke, the American geologist known for making the first oil discovery in Saudi Arabia in 1938, and then publicly again in the early 2000s by international oil companies, the basin appears to be of increasing interest once more.
“I would like to shed some light on the Rub’ al Khali, one of the world’s largest and youngest basins that is still forming,” said Misfir AzZahrani, executive director of Exploration for Saudi Aramco, prior to his February 2020 presentation at the AAPG Super Basin Conference in Sugar Land, Texas.
AzZahrani will also speak on “Geosciences in the Digital World: 2020 and Beyond” at the 14th Middle East Geosciences Conference and Exhibition in Bahrain this month.
Despite its rugged topography, the RAK has been mapped by Saudi Aramco to “acceptable” detail, most particularly its northern reaches.
“This region is part of one of the largest Arabian super basins, which has been receiving sediments for more than 500 million years and hosts a complete record of thick sediments from the Cambrian Age all the way to recent day,” AzZahrani said.
While the world’s largest conventional oilfield, the Ghawar, and the Shaybah gas fields comprise a small segment of the RAK, many other oil and gas fields have been discovered in the basin proper, he added.
Are more to come?
AzZahrani could not offer any details on recent or anticipated discoveries, but he confirmed that the basin is being explored. Publications by his peers and other geologists hint that RAK contains significant petroleum resources.
On the other side of the world in its Denver office, the U.S. Geological Survey is currently looking specifically at the RAK for a 2020 assessment of undiscovered unconventional resources. While exact numbers have not yet been published, preliminary data show significant promise and perhaps a clue into the emerging importance of the RAK as an unconventional basin.
RAK’s Geology
While not many publications exist on the structural geology of the Rub’ al Khali, a 2016 abstract in Tectonics, a peer-reviewed scientific journal of geology, written by Saudi Aramco Chief Explorationist Simon Stewart, helps to piece together a geological framework. Stewart obtained permission from the Saudi Arabian Ministry of Petroleum and Mineral Resources and from Saudi Aramco to publish the paper.
He stated that the RAK lies below a Quarternary sand sea, and its structural evolution from the Late Cambrian era to the Neogene period is only known through reflection seismic, gravity, magnetic data and well logs. The deepest structures imaged on reflection seismic data are undrilled Precambrian rifts filled with layered strata up to 13 kilometers deep.
Because of the lack of bedrock outcrops, the tectonostratigrahic evolution is only known from geophysical data and wells, according to Stewart.
Despite its harsh terrain, a “considerable amount” of subsurface data have been acquired in the pursuit of hydrocarbons, Stewart wrote, yet few analyses of its petroleum systems have been published.
Most striking to Stewart was a base Cambrian unconformity that is “by far the most significant break in the stratigraphy of the basin as imaged on reflection seismic data.”
He acknowledged two possible deep well penetrations in Precambrian rock that are consistent in orientation and spacing with Precambrian terranes exposed to the Arabian-Nubian Shield. Furthermore, in the center and east RAK, major folds developed episodically in the Late Cretaceous epoch and Tertiary period that Stewart called “economically significant because they host giant hydrocarbon accumulations.”
Yet, little else is known from the Arabian front.
Relying on published data, Chris Schenk, a USGS geologist, project chief for the National and Global Petroleum Assessment Project, is part of a team looking at resources in the RAK. The USGS is presently preparing an estimate of undiscovered, unconventional resources that could be eye opening.
“The source rock looks good,” said Schenk of the Silurian formation that has sourced the Paleozoic fields in the RAK.
Because of geologic uncertainly in the RAK, Schenk said further exploration is needed. However, based on his analysis, it appears that the majority of the RAK is in the gas window and could contain unconventional gas.
“The question is, as always, when in a frontier area, has the gas been retained with the shale or has it migrated?” Schenk said. “There are resources, but there is lot of geologic uncertainty.”
Significant Gas Potential
Saudi Aramco geologists Mohammad Fariqa, Abid Bhullar and Abdelghayoum Ahmed analyzed the Silurian Qusaiba shale play in a 2010 AAPG Search & Discovery presentation.
“The organic-rich shale of the Lower Silurian Qusaiba Member of the Qalibah Formation is one of the most prolific source rocks in the Arabian Peninsula. More than 90 percent of the Paleozoic light oil and gas accumulations are known to be sourced by this unit,” they wrote. “It is now under investigation as a potential shale gas play in several basins in Saudi Arabia.”
A 2014 publication titled “Geological Characteristics of the Lower Silurian Qusaiba Shale, Rub’ Al Khali Basin, Saudi Arabia” by Ayyaz Mustafa, Ali Sahin, Mohamed Abouelresh and Mustafa Hariri, concurs that the RAK has “significant potential for unconventional shale gas”.
In a 2018 article by Nadia al-Fawaz for the Middle Eastern publication, Al-Arabiya, geologist Hamoud al-Shanti poetically describes the windswept sands of the Empty Quarter and hypothesized that its subsurface is anything but vacuous. “Rub’ al Khali is a site that is rich in geological formations,” he said. “The Saudi government has not spared any effort to explore and conduct possible studies in an attempt to discover its secrets.”
It appears the world should stay tuned …
“Early explorers had their share of exploring this region,” AzZahrani said, “and today the search continues.”
The GEO 2020 conference and exhibition, previously scheduled for March 16-19 in Bahrain, has been postponed until Sept. 14-17, 2020. Although, having spoken to the authorities, the organizers are satisfied that the COVID 19 (Coronavirus) outbreak was intercepted and under very competent control, they are also mindful that many within our industry are currently facing difficulties traveling around the world.