GEO ExPro

Mr. Conjugate Margin

Early in his career, geologist Dr. Webster Ueipass Mohriak analysed 'the whole crust' along Brazil's continental margin to become one of the world's leading experts on conjugate margins.
This article appeared in Vol. 10, No. 2 - 2013

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Dr. Webster Mohriak worked for Petrobras before retiring in 2011 and becoming a consultant. He has authored numerous papers on salt tectonics and the petroleum exploration along the South Atlantic continental margins. Source: Thomas Smith

Webster was working in a bank in São Paulo, Brazil, at the age of 16 where he enjoyed writing international reports for the bank manager. The manager’s secretary translated German documents and had a ‘world of experience’ that impressed the young Webster, including experience in commercial analysis for the bank’s investment portfolio.

She sparked Webster’s interest in the sciences and encouraged him to ‘look to geology for good jobs and adventure’. Later that year he took the required exams to enter Brazil’s university system; the dreaded rite of passage, informally known as ‘the funnel,’ is a series of ferociously competitive exams that are must be passed before students can be accepted into a university. With only 50 slots in geology open to the country’s top-rated school, Webster secured part of his future by being accepted into the geology program at the University of São Paulo.

Inspiring Times

Subsequent to earning a degree in geology, Webster began working for Brazil’s state owned oil company, Petrobras, in 1980. After taking another national exam a few years later, he was picked by Petrobras higher management to study geology abroad. At Durham in north-east England, he started working on a PhD project under John Dewey. This move proved to be very influential in Webster’s development and geologic thinking. A year later, Dr. Dewey was appointed department head at the prestigious Oxford University where many of his students, including Webster, soon followed. At Oxford, “we (the students) worked in a basement across the road from the geology department,” says Webster. “Here, most of us acquired a cosmopolitan view of the geosciences, given the fantastic atmosphere of scientific research ongoing at Oxford with students from all over the world. The lectures, given by renowned geoscientists, inspired us to broaden our knowledge base from a local to a more regional approach.”

Indeed, in the late 1960s, John Dewey virtually grew up geologically with the then-newly accepted plate tectonics theory. While on sabbatical in New York, Dr. Dewey plotted the geologic details of the entire Caledonian orogenic belt that resulted from the closure of the Iapetus Ocean and the collision between north-western Europe and a North American-Greenland continent. The remnants of this once-prominent mountain range now curve from the eastern United States, up through Greenland, Norway, Scotland and Ireland. From these studies, he realized that there existed mountain belts of different types that could be explained only by the opening and closing of ocean basins.

In the early 1980s, concepts such as rifting and the ideas of existing conjugate margins were widely accepted by the geologic community but still poorly understood. One pioneering researcher, Bruce Rosendahl of Miami University, was trying to change our geologic understanding of rifts and continental margins and would become another major influence on Webster’s career.

While Webster was in graduate school at Oxford, Bruce Rosendahl was doing some of the first research on a nascent rift in East Africa, where great lakes such as Tanganyika and Malawi covered the deep structures beneath the water. Project PROBE took Rosendahl and his research vessel onto the great lakes of the East African Rift. He conducted seismic surveys along these lakes, making the first ever profiles of an embryonic rift system. Rosendahl was also one of the first to use ultra-deep, multichannel seismic techniques, in another PROBE Project, to image down to 40 km along the West African continental margin. The acoustic profiles offshore Gabon imaged the upper mantle, the rifted crust and the overlying sediments, revealing the segmented nature of how continents break up and how the South Atlantic Ocean was created.

Oceans Apart

While Bruce Rosendahl was looking at deep seismic records off the West African coast, a group of Petrobras researchers had their own ‘Crustal Project’ offshore Brazil. The Petrobras group, led by Webster after he finished his PhD, was analyzing the first deep seismic profiles acquired offshore Brazil.

This work would be pivotal for not only Webster, but in helping researchers realize the importance of looking at ‘the whole crust’ to understand the structure of Brazil’s most important producing basin since the 1980s, the Campos Basin. Again working with Petrobras, Webster and his fellow researchers soon became aware that the rift basins of eastern Brazil and West Africa would be better analyzed when integrated before the continental breakup and the formation of the South Atlantic Ocean that now separates the two continents.

“Once I realized how important it was to look past our own margins to understand the geology, I started exchanging technical communications with Bruce (Rosendahl) in the mid-1990s,” explains Webster. “He actually sent me several of the works he was doing with other colleagues that I used to analyze and compare with the continental margin work I was doing at Petrobras.”

Webster was now even more convinced of how important it was to see the big picture, even if some of the answers to questions lie across an ocean basin. He was driven to integrate and exchange knowledge from each side of the Atlantic Ocean margins. To this end, he organized one of the first sessions devoted to continental margins at the 5th International Congress of the Brazilian Geophysical Society held in São Paulo, Brazil, in 1997.

“This was one of the first technical meetings where the deep structure of the continental margins was analyzed and compared,” says Webster. “We had the opportunity to invite geoscientists from different research groups who were exponents in continental margin studies. The meeting exceeded all expectations and the American Geophysical Union proposed to have several of the presentations written as chapters for a book planned on continental margins. Atlantic Rifts and Continental Margins was then published, and it included some of the most referenced articles on West African margin geology.”

Unique Company Experience

Webster participating in field studies on an island off the Espirito Santo Basin (Abrolhos region), where he freely shares his knowledge of, and learns more about, the tectonics of continental margins. Source: Webster Mohriak “Petrobras is a company that provides incentives for its employees to search for solutions in-house by using the best science available worldwide,” explains Webster. “Specialists from many branches of the oil industry, from basic geology to ultra-sophisticated 3D visualization techniques, participate in projects either in the exploration or research groups. A tradition in the company is to have geoscientists and engineers visit classical basins around the world and participate actively in training programs abroad while working with other renowned specialists. This way, some of our solutions for geoscience and engineering questions have come from the analysis of worldwide analogs.”

Webster explains that, “For nearly 30 years, the focus at Petrobras has been on turbidites as the main reservoirs for fields found along the continental margins, not only in Brazil but in many other exploration areas. Now, the company has shifted toward the pre-salt carbonates which are becoming the primary target in deepwater drilling. This has led to research projects and PhD studies in Brazil and abroad. Again, the company and several of the exploration partners are applying the best technologies available in order to fully unlock the production capacity of these carbonate (microbialites) rocks.”

Webster’s own experience at Petrobras reflects the company’s philosophy of applying the ‘best’ science available to their producing basins. He started out in the 1980s working on a project aimed at analyzing the factors that controlled the occurrence of hydrocarbons in the Campos Basin. “We investigated why this relatively small basin was so prolific when compared to other basins,” says Webster. “The answers started to appear by integrating all the data. This basin has a very rich source rock, an evaporite layer that created both seals and windows for migration from the pre-salt source rocks, and a Late Cretaceous to Tertiary uplift forming coastal mountains, the erosion of which deposited widespread turbidites into the deep waters of the basin.”

At this time, Giuseppe Baccocoli (see GEO ExPro Vol. 5, No. 4), a senior Petrobras geologist renowned for early studies in the Campos Basin that led to the first commercial discovery in that basin, inspired Webster to learn more about the salt layer and its deformation. Webster was asked to analyze one of the first AAPG volumes on salt diapirs in the Gulf of Mexico. This seeded a particular interest in his career, which was devoted to understanding salt tectonics in the Campos Basin.

Webster furthered his research and understanding of continental margins with Petrobras in the early 1990s by participating in the Cabo Frio Project. “This would be the first project to integrate the regional datasets from the southern Campos and northern Santos basins,” Webster explains. “We identified the Cabo Frio fault zone that extends from the central Santos Basin into the Campos Basin. The fault zone is a major antithetic fault that is associated with salt flowing basinwards above a regional high. This created one of the largest structures in the South Atlantic, ‘the Albian gap,’ a region devoid of the Albian carbonates. The study was presented at an international conference on salt tectonics, in Bath, England, in 1993. Specialists were amazed with that intriguing structure and the paper is still considered one of the landmarks on salt tectonics. As a matter of fact, several research institutions worldwide have tried to simulate physically and numerically the development of that structure.”

Another example of ‘applying the best science’ occurred at a meeting in Houston in the early 1990s, the GCSSEPM conference ‘Salt, Sediment and Hydrocarbons.’ Webster attended the presentation on the Mahogany discovery in the Gulf of Mexico, the first sub-salt commercial find. “This made us wonder if we had similar structures offshore Brazil,” Webster recalls. “We came away from the conference analyzing some deep blocks in the Espirito Santo Basin. Several years later we interpreted an allochthonous salt tongue with possible Tertiary reservoirs below the Aptian salt, which we called a sub-salt play. Similar structures have proven productive off Africa and, who knows, we may find out in the coming years if the structures off our coast will bear hydrocarbons as well. This is just one more example of how regional studies and analogs from different areas can be used for exploring less known or even frontier regions elsewhere in the world.”

International Focus

Since his recent retirement from Petrobras in 2011, as a consultant Webster has continued to apply his vast knowledge of continental margin geology to places like the Solimões Basin in the upper Amazon region of Brazil and to new exploration off the coast of Namibia, West Africa. “I am working to integrate datasets in order to propose viable models for future prospects,” says Webster. “We [geoscientists at HRT Oil & Gas] are fine tuning our structural models for known fields to be used as analogs for structures to be drilled. One of those, the Castanheira prospect, resulted in one of the best gas discoveries in the Solimões Basin.”

He is also pursuing more academic collaboration by contributing as one of the editors and co-authors in a book that will be published by the Geological Society of London in 2013, Conjugate Divergent Margins. In one chapter in the publication, Webster and his co-author make comparisons between the Red Sea-Gulf of Aden and the South Atlantic.

I finally met Webster in 2012 at the most recent Perkins Conference in Houston, entitled New Understanding of the Petroleum Systems of Continental Margins of the World. He was certainly in his element, conversing with experts who study the continental margins of the Gulf of Mexico, Australia and New Zealand, South America, Africa, Europe and Asia. Our interview was punctuated by interruptions from other well-wishing geoscientists. It was clear Webster’s focus for the future is to expand the existing knowledge of basins and their petroleum systems around the world and encourage mutual collaboration. He truly believes that:

“Allowing university geoscience students to work with Brazilian datasets as well as data from the West African margin and ongoing projects involving international teams with specialists from Germany, England, United States and many other countries, will result in a better understanding of the petroleum systems for the rift basins. This and future cooperative efforts will usher in a new stage of exploration when the concepts developed in one segment of the margin can be immediately applied to other segments across an ocean.”

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