The rule of Uniformitarianism gives confidence in picking well sites through looking in ~ Holocene facies analogues.

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The tidal flats and reefs the the Red Sea checked out from the air – a great way of putting a reservoir analogue into perspective. (Source: ann Whaley Sousou)The rule of Uniformitarianism says that the causes and also rates of geologic processes that are in operation today space the same as those the existed in the geologic past. Reefs law the exact same today as they go in the past, back their key frame-builders have advanced through time. Sand bars and tidal level settings also behave together they have actually in the past. Much more simply stated, the principle states that ‘the existing is the an essential to the past.’ This short article describes several instances from the modern-day world, where primary patterns the sedimentation provide tremendous understanding into subsurface distribution of reef, sand bar and tidal flat settings – the 3 principal species of depositional setups for hydrocarbon reservoirs that produce from carbonate rocks. 

Methodology 

Think the a modern-day sediment map together a time slice; it simply happens to be the recent one – the Holocene. To make such a map, bottom samples need to be gathered and analysed for numerous properties, such as the portion of mud, sand and also gravel, the variety of non-carbonate material in each dimension fraction, and also the kind of seed observed. A shorthand system of signs is supplied to alleviate these data come mappable entities: Sediment name = (sediment descriptor) (dominant grain type) (secondary grain type) (texture) + (remark) An instance of this device would be:

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which converts to head-coral mixed-skeletal grainstone through branching corals. Other examples include ooid grainstone: 

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and algal stromatolite boundstone: 

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This notation treats sediments as if they were rocks, making comparison v subsurface lithologies and also patterns more direct and also meaningful. 

NB In the maps displayed in this article, red suggests reef facies, and also yellow suggests shoals. Other colours are used for various carbonate facies, and also shades that grey are offered for shales. 

Example 1: 

A patch Reef Complex, Indonesia 

300 bottom samples were accumulated from Kepulauan Seribu, a chain that islands phibìc of Jakarta (Jordan, 1998). Depositional environments and also sediments right here consist of the adhering to types: Island (brown): 

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coral-fragment mixed-skeletal grainstone. Reef facies (red): 

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head-coral encrusting-red-algal boundstone. 

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with a matrix of coral-fragment mixed-skeletal packstone occurring in between reef frame-builders. Near-reef facies (yellow): 

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coral-fragment mixed-skeletal grainstone v branching-coral fragments, and 

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coral-fragment molluscan packstone through branching-coral fragments.Reef-sediment slope (light blue): 

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coral-fragment molluscan packstone. Inter-reef lagoon (light grey): 

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argillaceous benthonic-foram molluscan packstone through goethite nodules.

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Middle shelf (dark blue): 

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arenaceous foram molluscan shale with goethite nodules.

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Sedimentologic map of Kepulauan Seribu, IndonesiaThese varieties of coral-reefs and also associated archipelago are typical throughout Indo-Pacific locations today. Reefs act together factories, exporting sediment in a radial pattern far from the reef centre. Big volumes of carbonate sediment form noticeable concentric white sand rims in near-reef and also reef-sediment-slope environments, together observed in satellite images and in usual air photos. Red areas on the Kepulauan Seribu map show a rim of living reef, farming up come sea level, the surrounds every individual platform. A preferably depth of about 40m wake up in inter-reef lagoons between platforms. Note that reef communication are biggest in the southern component of the reef complicated and smaller sized to the north, reflecting greater subsidence to the north, where reefs space struggling to save up v a relatively rising sea level. Yellow-coloured areas dominate this map, showing an ext parts room covered by reef-derived sediment than by the reef itself. This says that in old settings the drill little bit is more likely not to penetrate true reef facies, however near-reef sediments instead. This map compares well through that that the huge Arun gas field of Miocene age in the north Sumatra Basin. The carbonate settings of the two areas show comparable platform shapes and also sizes and also have comparable coral faunas, similar facies diversity, comparable forams, comparable subsidence patterns, and even the same windward side (the east side). Right here one have the right to swim now over the same types of corals that take place 3,000–3,350m deep at Arun, so over there is no doubt about the environment of deposition because that Miocene reef builders. 


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The corals watched growing around the Indonesian Kepulauan Seribu chain the islands space the same varieties as the Miocene corals found at the giant Arun field. (Source: Richard Bowden)


Example 2: 

Barrier Reef System, Belize Shelf 

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Sedimentologic map the Belize, main AmericaOver 800 samples comprise the data base because that the sedimentologic map that the Belize Shelf in central America, which displayed a barrier reef system and also atolls. Again, red is reef, and yellow to represent grainstone shoals. The map reflects a wide range of muddy sediment types on a vast shelf behind the Belize obstacle Reef, i m sorry is about 180 kilometres long, making the the 2nd longest barrier reef in the world. Barrier reefs likewise rim 3 offshore atolls; job reefs take place behind the barrier to the west and also in the centre of the atolls. A blended carbonate-clastic shelf wake up behind the barrier, and the northern and also southern shelves have significant differences. Nearshore clastics occur along the shore of the southern shelf, supplied by rivers flowing eastern from the Maya Mountains, and do not take place on the northern part. The southern fifty percent of the shelf has much more topographic relief, with an ext middle-shelf patch reefs, and also is more comprehensive than the north half, i m sorry is a southern expansion of the fairly flat Yucatan Plateau. The broad of the southern shelf is around 50 km, as contrasted with much less than 1.5 km large on the northern shelf in prior of san Pedro, the main tourist attraction. Much less patch reefs, but an ext shoals, such together foram grainstone sand bars or huge mud-mounds, take place on the northern shelf. Normal naval salinities happen throughout the southern shelf, conversely, the big embayment in ~ the north end of the Belize Shelf, Chetumal Bay, has substantial freshwater dilution and reduced salinities, which room reflected in the restricted faunas there – no corals, no platy green algae, only molluscs and a restricted foram fauna. This map reflects a shelf through a wide selection of middle-shelf sediments, a drop-off shelf margin (as compared to a ramp profile), a restricted or clastic inside shelf and also several middle-shelf reef patterns, and two species of obstacle reefs. Several areas of this map to compare well with maps of oil and also gas fields in upper Palaeozoic rocks the west Texas. 

Example 3: 

Tidal Flats, Arabian Gulf 

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Sedimentologic map that the Arabian Gulf.Sedimentologic data the Purser (1971) were field checked, reformatted, and used to develop a sedimentologic map that the entire Arabian Gulf, i beg your pardon is overcame by tidal level deposits, ooid bars, reefs and shell banks. The local setting is just one of a ramp profile, and also the deepest component of the Gulf is no in the middle, but is counter to the north, closer to Iran. Sediments throughout the gulf are conquered by muddy carbonates with numerous molluscs, the an excellent Pearl financial institution being the most renowned example. Reefs (in red) are not common and are usually best developed as local attributes on optimal of salt domes. Ooid grainstone shoals (in yellow) are very rare, emerging as local tidal delta deposits turn off the Abu Dhabi coastline north-west of the funding city. What the Arabian coast is most renowned for space its extensive, well-developed tidal flats, consisting of algal-stromatolite boundstones through gastropods. These deposits carry out excellent insights into processes of dolomite formation. The finest modern-to-ancient comparisons of these tidal flats room the giant areas of the Jurassic Arab zones in Saudi Arabia, wherein each the the Arab A, B, C, and D cycles room capped by together deposits. 

Example 4: 

Oolites and also Reefs, an excellent Bahama Bank 

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On this local sedimentological map the the an excellent Bahama Bank, one have the right to see four different models for the development of ooid bars: 1) Schooner Cays and also southern finish of Tongue of the ocean (tidal bar belts at the outer shelf), 2) Joulters Cay (a shelf-edge paper deposit that includes ooid packstones), 3) the Exuma archipelago (narrow passes between paleotopographic high at the shelf edge), and also 4) the shelf-edge Cat Cay ooid bar southern of Bimini, which procedures several 10s of kilometre long.Hundreds that samples administer the basis for the sedimentologic map that the great Bahama Bank, which is composed of several types of oolite bars, plus tidal flats and also reefs. Numerous researchers have contributed to this understanding, but the name of Ginsburg was standing out over the rest. The an excellent Bahama financial institution is the biggest carbonate platform in the world, through a drop-off file on all sides. That is shelf-margin deposits consist one of two people of ooid grainstones (yellow areas) or branching-coral encrusting-red-algal boundstones (red areas). Middle-shelf sediments consist of vast expanses that muddy carbonate facies however include some neighborhood sand shoals and also patch reefs. Inner-shelf sediments host algal-stromatolite boundstones, developed on tidal flats, especially those follow me the west coast of Andros Island, the biggest island ~ above the platform. Underlying Pleistocene topography, especially aeolianite deposits, substantially influences the distribution of Bahamian Holocene sediments. Zooming in on the local sedimentological map one can see four various models because that the development of ooid bars. Other local locations of interest encompass the barrier reef along the east coast of Andros Island, the third-longest barrier reef in the world, with a narrow shelf west that it, consists of bones sands and tiny patch reefs. An additional area of in-depth study is the Turks and also Caicos, which shows a well occurred outer-shelf barrier reef only on that is northern, windward side; skeletal sands (with no job reefs) take place on the southerly side the this currentswept, secluded platform and also has oceanic depths neighboring it on every sides. 

Anyone for a ar Trip? 

There is nothing quite prefer seeing rocks and sediments in the field, therefore to make these to compare personally, the reader is invited to participate in upcoming field trips readily available by the writer (www.carbonaterocks.com). The key objective is to present living instances of reservoir facies encountered by the drill bit. The is an enjoyable means to learn and also has the added benefits of team-building. The is especially helpful to take a trip over an area, so that the scale and ‘map view’ room put right into perspective, particularly the odds that drilling right into targeted facies belts.