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Bulletin Article

Geothermal convection in South Atlantic subsalt lacustrine carbonates: Developing diagenesis and reservoir quality predictive concepts with reactive transport models

Prolific hydrocarbon discoveries in the subsalt, commonly known as the “presalt,” section of Brazil and the conjugate African margin have created a business imperative to predict reservoir quality in lacustrine carbonates. Geothermal convection is a style of groundwater flow known to occur in rift settings, which is capable of diagenetic modification of reservoir quality. We simulated variable density groundwater flow coupled with chemical reactions to evaluate the potential for diagenesis driven by convection in subsalt carbonates. Rates of calcite diagenesis are critically controlled by temperature gradient and fluid flux following the principles of retrograde solubility. Simulations predict that convection could operate in rift carbonates prior to salt deposition, but with rates of dissolution in the reservoir interval only on the order of 0.01 vol. %/m.y., which is too low to significantly modify reservoir quality. The exception is around permeable fault zones and/or unconformities where flow is focused and dissolution rates are amplified to 1 to 10 vol. %/m.y. and could locally modify reservoir quality. After salt deposition, simulations also predict convection with a critical function for salt rugosity. The greatest potential for dissolution at rates of 0.1 to 1 vol. %/m.y. occurs where salt welds, overlying permeable carbonates thin to 500 m (1640 ft) or less. With tens of million years residence times feasible, convection under these conditions could locally result in reservoir sweet spots with porosity modification of 1% to 10% and potentially an order of magnitude or more in reservoir permeability. Integrating quantitative model–derived predictive diagenetic concepts with traditional subsurface data sets refines exploration to production scale risking of carbonate reservoir presence and quality.

Added on 31 July, 2013

Bulletin E-P Note

Seismic geomorphological analysis and hydrocarbon potential of the Lower Cretaceous Cromer Knoll Group, Heidrun field, Norway

The Heidrun field, located on the Halten Terrace of the mid-Norwegian continental shelf, was one of the first giant oil fields found in the Norwegian Sea. Traditional reservoir intervals in the Heidrun field lie within the Jurassic synrift sequence. Most Norwegian continental shelf fields have been producing from these Jurassic reservoirs for the past 30 yr. Production has since declined in these mature fields, but recently, exploration for new reservoirs has resurged in this region. The Jurassic rifted fault blocks form a narrow continental shelf in Norway, thereby greatly reducing the areal extent for exploration and development within existing fields. As the rift axis is approached farther offshore, these Jurassic reservoirs become very deep, too risky to drill, and uneconomical. This risk has prompted exploration in more recent years of the shallower Cretaceous, postrift stratigraphic succession. Cretaceous turbidites have been found in the Norwegian and North Seas, and the discovery of the Agat field in the Norwegian North Sea confirms the existence of a working petroleum system capable of charging Cretaceous reservoirs. These Cretaceous reservoirs were deposited as slope- and basin-floor fans within a series of underfilled rifted deeps along the Norwegian continental shelf and are thought to be sourced from the localized erosion of Jurassic rifted highs. We use three-dimensional seismic and well data to document the geomorphology of a deep-water, Lower Cretaceous wedge (Cromer Knoll Group) within the hanging wall of a rift-related half graben formed on the Halten Terrace offshore mid-Norway. Seismic attribute extractions taken within this Lower Cretaceous wedge reveal the presence of several lobate to elongated bodies that seem to cascade over fault-bounded terraces associated with rifted structures. These high-amplitude, elongated bodies are interpreted as deep-water sedimentary conduits that are time equivalent to the Cretaceous basin-floor fans in more distal parts of the basin to the west. These half-graben fills have the potential to contain high-quality Cretaceous sandstones that might represent a potential new reservoir interval within the Heidrun field.

Added on 31 July, 2013

Explorer Historical Highlights By James Pindell

Origin of the Caribbean? Look Toward the Pacific

Were there enough arguments to champion a firm stand for a Pacific origin of the Caribbean lithosphere, as Kevin Burke, Bruce Malfait and others had suggested?

Added on 01 July, 2013

Explorer Geophysical Corner By Evan Staples,Kurt Marfurt,Satinder Chopra

Testing Curvature's Impact on Fractures

Fracture zones can be critical to improving or creating sufficient porosity and permeability in hydrocarbon reservoirs – with strain, along with lithology and thickness being the major controls.

Added on 01 July, 2013

Explorer Emphasis Article By Louise Durham

Geomechanics Helps Solve Horizontal Woes

Think fast: A new risk-based approach to geomechanics is being used to help solve horizontal drilling problems.

Added on 01 July, 2013

Bulletin Article

Influence of fault rock foliation on fault zone permeability: The case of deeply buried arkosic sandstones (Gres d'Annot, southeastern France)

We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz and calcite cements demonstrates fault activity at temperatures of 195C to 268C. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins) is the direction of highest permeability in the foliated sandstone (10–2 md for Y against 10–3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particles forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault and subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions.

Added on 30 June, 2013

Bulletin Article

Linking process, dimension, texture, and geochemistry in dolomite geobodies: A case study from Wadi Mistal (northern Oman)

Understanding the distribution and geometry of reservoir geobodies is crucial for net-to-gross estimates and to model subsurface flow. This article focuses on the process of dolomitization and resulting geometry of diagenetic geobodies in an outcrop of Jurassic host rocks from northern Oman. Field and petrographic data show that a first phase of stratabound dolomite is crosscut by a second phase of fault-related dolomite. The stratabound dolomite geobodies are laterally continuous for at least several hundreds of meters (1000 ft) and probably regionally and are one-half meter (1.6 ft) thick. Based on petrography and geochemistry, a process of seepage reflux of mesosaline or hypersaline fluids during the early stages of burial diagenesis is proposed for the formation of the stratabound dolomite. In contrast, the fault-related dolomite geobodies are trending along a fault that can be followed for at least 100 m (328 ft) and vary in width from a few tens of centimeters to as much as 10 m (1–33 ft). Petrography, geochemistry, and high homogenization temperature of fluid inclusions all point to the formation of the dolomite along a normal fault under deep burial conditions during the Middle to Late Cretaceous. The high 87Sr/86Sr ratio in the dolomite and the high salinity measured in fluid inclusions indicate that the dolomitizing fluids are deep basinal brines that interacted with crystalline basement. The dolomitization styles have an impact on the dimension, texture, and geochemistry of the different dolomite geobodies, and a modified classification scheme (compared to the one from Jung and Aigner, 2012) is proposed to incorporate diagenetic geobodies in future reservoir modeling.

Added on 30 June, 2013

Bulletin Article

A transition from carbonate shelf to pelagic basin environments of deposition: Rifting and depositional systems in the Jurassic of northeastern Tunisia

Jurassic deposition in the Maghrebian tethys was governed by eustasy and rifting. Two periods were delineated: (1) a carbonate shelf (Rhaetian–early Pliensbachian) and (2) a platform-basin complex (early Pliensbachian–Callovian). The carbonate shelf evolved in four stages, generating three sedimentary sequences, J1 to J3, separated by boundary sea level falls, drawdown, exposure, and local erosion. Sediment facies bear evidence of sea level rises and falls. Lateral changes in lithofacies indicate shoaling and deepening upward during the Sinemurian. A major pulse of rifting with an abrupt transition from carbonate shelf to pelagic basin environments of deposition marks the upper boundary of the lower Pliensbachian carbonate shelf deposits. This rifting episode with brittle fractures broke up the Rhaetian–early Pliensbachian carbonate shelf and has created a network of grabens, half grabens, horsts, and stacked ramps. Following this episode, a relative sea level rise led to pelagic sedimentation in the rift basins with local anoxic environments that also received debris shed from uplifted ramp crests. Another major episode spanning the whole early Pliensbachian–Bajocian is suggested by early brecciation, mass flows, slumps, olistolites, erosion, pinch-outs, and sedimentary prisms. A later increase in the rates of drifting marked a progress toward rift cessation during the Late Jurassic. These Jurassic carbonates with detrital deposits and black shales as the source rocks in northeastern Tunisia may define interesting petroleum plays (pinch-out flanking ramps, onlaps, and structurally upraised blocks sealed inside grabens). Source rock maturation and hydrocarbon migration began early in the Cretaceous and reached a maximum during the late Tortonian–Pliocene Atlassic orogeny.

Added on 30 June, 2013

Bulletin Geologic Note

A new theoretical approach to model sorption-induced coal shrinkage or swelling

The shrinkage or swelling of coal as a result of gas desorption or adsorption is a well-accepted phenomenon. Its impact on permeability changes has also been recognized for two decades. Its importance has increased significantly because of the potential of coals that are not likely to be mined and depleted or nearly depleted coalbed methane reservoirs to serve as CO2 repositories. This article proposes a new theoretical technique to model the volumetric changes in the coal matrix during gas desorption or adsorption using the elastic properties, sorption parameters, and physical properties of coal. The proposed model is based on the theory of changes in surface energy as a result of sorption. The results show that the proposed model is in excellent agreement with the laboratory volumetric strain data presented in the literature during the last 50 yr. Furthermore, the proposed model can be extended to describe mixed-gas sorption behavior, which can be applied to enhanced coalbed methane and CO2 sequestration operations.

Added on 30 June, 2013

Explorer Emphasis Article By Louise Durham

Diverse Dynamics Impact Bakken Productivity

Complex considerations: Mention the Bakken Formation and most people think of unlimited potential – but several dynamics have a huge impact on productivity.

Added on 01 June, 2013

Hot Items

Explorer Article By Emily Llinás

Offshore Opportunities in Suriname

The South American country of Suriname has launched an aggressive campaign to ramp up offshore oil production.

Added on 01 April, 2024

Explorer Director’s Corner By David Curtiss

Introducing the New EXPLORER, New Digital Newsletters

Perhaps you did a double take pulling the April issue of EXPLORER from the mailbox. What is this? If you joined AAPG in the last 40 years, you’ve only known EXPLORER in its long-standing tabloid format. It worked well for many years as our advertisers – particularly seismic companies – loved the large format and the ability to display their data on a sweeping canvas. For readers, it was a little more awkward.

Added on 01 April, 2024

Explorer Article By David Brown

Persistent Prospects of the Permian

An influx of recent mergers and acquisitions has brought even more operators to the Permian, and there are some regions where there is still much geoscience to be done.

Added on 26 March, 2024

Explorer Article By Emily Llinás,Sylvia Anjos

The ‘Father of the Pre-Salt’

Brazil’s famed “Father of the Pre-Salt,” Guilherme de Oliveira Estrella, is credited with several major oil discoveries and with advances in petroleum systems research.

Added on 01 April, 2024

Explorer Article By Jhonny Casas

Angelo Pitoni, a Real-Life 'Indiana Jones' in Venezuelan Territory

Adventure enthusiasts should enjoy the story of Angelo Pitoni, a larger- than-life geologist whose story includes exploits rivaling any of those portrayed on the big screen. This article focuses specifically on a chapter of his life that took place in my country, Venezuela, and forms part of our country’s oil exploration history.

Added on 01 March, 2022

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