Friday, February 2, 2018

Structural Interpretation in Offshore Congo

Petroleum exploitation begins with geologist and geophysicst exploring unknown territory with few wells for guidance and no visible sign of oil and gas. Their task is to uncover the best prospects and decide whether oil companies should commit to millions of exploration dollars. A key tool is structural interpretation of seismic data. A survey shot in deep waters off the Congo mainland shows how geologist apply this technique. 

 Play in exploration context describes a geological configuration that favors the accumulation of hydrocarbons. Plays and their associated prospects are what exploration geologist sped most of their professional lives looking for.

 Until oil or gas is discovered, plays exist mostly in the mind. Focusing on one or another of the earth's basins, explorationists search for that seemingly impossible concatenation- source rock, migration path, reservoir and seal. Each vital ingredient must be present, both at the correct physical location and at the right time.

 Source rocks rich enough in organic material must have been buried and heated to sufficiently high temperature and long enough to form petroleum. Petroleum migrates upward, so there must have been a conduit to guide it. And a porous, permeable reservoir rock capped by impermeable rocks is required to receive and trap the fluid. Finally, the ensuing geological evolution - commonly several tens or even hundreds of million years - must have left the reservoir and seal intact. 

As explorationists focus on a sedimentary basin, this juxtaposition of geological coincidence occupies the mind. Every scrap of evidence is used to refine the notion of how the basin might have evolved and whether the structural and sedimentary history might favor a play. Outcrop geology, satellite imagery, magnetic and gravity surveys, and especially seismic data contribute to the interpretation process. This article shows how various plays in offshore Congo crystalized in the minds of geologists as they reviewed a nonproprietary seismic survey shot in West African waters.

Geologist first became interested in the Congo in 1928, attracted by tar seepages near Pointe Noire on the coast and knowledge that the subsurface contained thick deposits of salt- salt intrusions provide a classic trapping mechanisms for hydrocarbon accumulation. The salts had been identified in numerous boreholes drilled for potash mining exploration.

Oil was discovered at Pointe Indienne, 20 kilometers north of Pointe Noire, in 1959. But further onshore exploration proved fruitless - some oil shows , but nothing of commercial value. In the late 1960s, an Elf/AGIP partnership discovered oil offshore. Further successes yielded five offshore fields and oil production  from the Congo now exceeds 115,000 barrels per day. Recoverable reserves are currently estimated at more than a billion barrels. 

The GECO_PRAKLA nonproprietary survey of 1990, which forms the basis of the interpretation described in this article, covered 15,000 square kilometers. The survey ventured from shallow water at the edge of the continental shelf, where oil plays were known to exist, to depths up to 2000 m [6560 ft]. This is deeper than the current limit of commercial exploitation, but the deepwater data were nevertheless crucial for elucidating basin structure and migration pathways from deep in the basin. The survey lines were designed to connect with previous surveys conducted closer to the coast and intersect any exploration wells where precise lithological data would be available.

The offshore basins of the Congo form part of the West African Salt Basin, a large collection of basins stretching 2000 km from south Cameroon to Angola. The salt was deposited during Aptian times about 120 million years ago when the rifting of South America from West Africa gradually evolved into a full-fledged drift. Later salt movement would create anticlinal folds capable of trapping hydrocarbons. 

The story , begins thirtly million years earlier during the Late Jurassic. At this time, extensional faulting and subsidence took place in the part of the Gondwana supercontinent that would eventually become both the east coast of South America and the west coast of Africa. Further stretching or extension in the Early Cretaceous led to the formation of a large-scale rift along the future western Africa and eastern Brazilian margins. A modern parallel is the Great Rift Valley extending from the Red Sea to the Zambesi River.

Initially, the rift and its basins were above sea level and isolated from the ocean. Large lakes formed in which sandstone and shales accumulated. Some shales were deposited in oxygen-deficient water, allowing preservation of organic matter. These formations are the source for billions of barrels of oil found in the West African basin. 

By Aptian times, continued subsidence and a rise in global sea level permitted incursion by the sea. At first, this was intermittent, with the sea alternately entering and receding from the basins. This created ideal conditions of repeated evaporation and marine flooding to create thick deposits of halite, the Aptian salts.

Later, during the mid to Late Cretaceous, the area was definitely submerged and continental breakup of Gondwana led to a separation, or drift, of South America from Africa. Whreas the basins had previously been linked on one continental plate, now they were separated by a widening tract of ocean, as the Atlantic opened through injection of new oceanic crust at the mid-ocean ridge. Sedimentation was now marine, with thick deposits of limestone, sandstone and shale. Further subsidence took place in the Late Tertiary and was probably associated with faulting related to the collision of the Eurasian and African plates.

This big picture the interpreters learned from extensive research into the region's geological history. Another card in their hand was knowing which formations had produced oil shows during drilling in the area , which of these had produced commercially, and which formation was the most likely source for the shows - the most important was the Marnes Noires, or black marls, continental deposits formed under lacustrine conditions late in the rift phase and prior to the invasion of sea water. 

The majority of commercial reservoirs in offshore Congo occurs in sandy and dolomitic rock deposited during the early drift phase in the mid-Cretaceous, with structural traps created by movement of the transition-phase Aptian salts. Noncommercial oil shows have been found aplenty in rift and transition rocks of the Lower Cretaceous , but no oil has been tested in Upper Cretaceous or Tertiary formations.

The interpreters knew the source of oil, therefore, and in general how it most  likely migrated and became trapped. What they did not know -and what they sought during the interpretation - was information on the three-dimensional structure of the deepwater Congo sediments -specifically the location, general distribution and size of likely hydrocarbon prospects. Their immediate goal was to identify target areas suitable for detailed mapping with more closely spaced surveys. 

We rejoin our interpreters as they inspect the survey's processed and migrated sections and begin the critical task of identifying formation tops - the lenghty data processing was previously accomplished.


 Scanning the sections, the interpreter will also note major structural features such as listric faulting and salt bodies, and begin the highly skilled task of constructing a picture of the subsurface, all the time drawing and updating conclusions on a working map. Perhaps the interpreter's most cherished skill is this ability to visualize in three dimensions.

The basis of structural seismic interpetation is the loop method, in which a seismic reflector representing a geologic horizon is mapped around a series of intersecting sections and then back to original section.Closing the loop is easier said than done. Tracing the continuity of a reflector can be tricky across a fault and sometimes impossible if the formation pinches out laterally or has been eroded to form an unconformity. Problems may also occur in areas of steep dip, where 2D migration on each section fails to image complex three-dimensional structure correctly, producing a slight mistie.

Typically, the interpreter plots the major geological units from the logs on the seismic section at the well location to obtain the best correlation. Lithology changes observed on the logs assist the correlation process. For example, a slow formation such as mudstone overlying a fast formation such as tigh limestone typically correlates with a white band or trough on the seismic section. Conversely, a fast formation overlying a slow formation generally correlates with a black band or peak.  

Once the best correlation is found, the interpreter colors the section at the well location according to the geological units on the log.

Some interpreters identify all faults on the section (red marking) and then track reflector continuity. Others may concentrate on reflectors and reflector terminations- onlaps and truncations - concentrating on the stratigraphy and marking in only those faults that bear on the work at hand.

In both example sections, the thin Aptian sand (orange) forms a boundary between the rift sediments below and the drift sediments above. Overlying the sandstone, the salt (purple) intrudes into the overlying sediments, creating structural traps for oil generated in the Marnes Noire below.

The first section that lies parallel to the direction of geological dip cleary shows fault blocks in the deep rift sediments and large-scale listric faults in the shallower drift sequences. A listric fault has a pronounced curved slip face. In this example, the sediments to the left (southwest) of the fault have been displaced downward and rotated clockwise. 

The interpretation seems clean and finished, yet questions often remain. Formation tops may be uncertain, particulary in the deeper section beyond the range of well control and where seismic data lose their resolution. The exact shape of the all important salt intrusions may be subject to different interpretations. A solution for resolving these cases lies with magnetic and gravity data, acquired concurently with the seismic data. 

At this point, all the factors necessary to pinpoint plays are at hand. The burial history confirms the potential of organic-rich horizons as source. Detailed comprehensive and mapping of basin structure reveals likely migratory paths and trapping mechanisms. The thickness maps indicate the distribution and geometery of sediment bodies and assists in recognizing commercially interesting reservoirs. Deciding the location of plays now demands of the interpreter a juggling of these factors and the picking of locations and depths where all indications appear simultaneously favorable. The result is a play map that oil companies can use to decide

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