Fault-seal analysis lessens ambiguity in the Sinjhoro area, Pakistan

June 5, 2017
The Lower Goru is a prolific producing reservoir in Pakistan's Sinjhoro area in the Lower Indus basin. Fault-seal analysis can confirm hydrocarbon traps, and a new study has lowered uncertainty by categorizing two faults in the region based on wall-rock juxtapositions, Allan diagrams, shale gouge ratios (SGR), and shale smear factors (SSF).

Gulraiz Akhter Bilal Aslam
Anees Ahmed Bangish
Quaid-i-Azam University

The Lower Goru is a prolific producing reservoir in Pakistan's Sinjhoro area in the Lower Indus basin. Fault-seal analysis can confirm hydrocarbon traps, and a new study has lowered uncertainty by categorizing two faults in the region based on wall-rock juxtapositions, Allan diagrams, shale gouge ratios (SGR), and shale smear factors (SSF).

The Lower Goru sands consist of many productive zones-interbedded sands and shale sequences-and similar studies regarding the region's basal sands and bounding faults could enhance the region's production.

Fault analysis

Petroleum systems are primarily associated with fault-bounded structural traps, which may act as a seal or conduit for hydrocarbon flow. Fault-seal analysis typically involves constructing fault-plane displacement and juxtaposition maps, and identifying fault-membrane seals. Several studies have addressed the importance of fault-seal analysis in oil and gas exploration (OGJ, Mar. 22, 2010, p. 34).1 2

Misidentification of fault seals can negatively impact exploration and development projects. To minimize this risk, reservoir engineers attempt to categorize faults based on juxtaposition and clay smear or SGR analysis. This article interpreted multiple faults on depth contour maps but selected two faults-F1 and F2-on which to perform fault seal analysis based on positive depth-contour leads. Both faults bound a previously drilled well and may act as a seal.

Indus basin geology

The Lower Indus basin features a passive roof complex-type structure and a passive back thrust along the Kirthar foldbelt.3 The basin is bordered by the Indian Shield to the east and the Kirthar range to the west. The basin is bounded north and south by the Jacobabad Khair Pur High and the offshore Murray Ridge plate, respectively.4 The Sinjhoro area comprises normal and strike-slip faulting similar to nearby fields (Fig. 1).

Fault-throws could be separating sand reservoirs in the field, and the Tertiary faults generated from crustal plate bending are likely due to collision and tensional release.5 These characteristics are extensional features based on previous seismic studies and fault-plane analyses.6 Extensional tectonic forces formed the tilted fault blocks over a wide area of the eastern Lower Indus sub-basin during the Cretaceous.7 Normal faulting across the complete southern basin has produced horsts and graben structures important for further exploration.

A system of faults with normal dip separation breaks Cretaceous and older layers. The Cretaceous faults generally strike between N 30° W and N 50° W, arranged in an echelon formation in zones trending near N-S.

The tilted fault block traps show continuation while hydrocarbons were being generated. Fault-associated structural closures trapped oil and gas in Lower Goru sandstone in the Sinjhoro block. Under filling is attributed to upward leakage across the structures that redistributed the hydrocarbons.8

Lower Goru sandstone is the probable reservoir rock in the area. All producing wells in the study area tap Type 3-Kerogen source rock (Fig. 2). The Upper Goru shale provides the local seal for the Cretaceous Lower Goru.7 The reservoir contains a stratigraphic trapping arrangement with the transgressive shales of the Lower Goru C interval directly overlaying the reservoir sands. Thick shales and marls of the Lower Goru member form the regional top seal. Shales and tight sands in the C interval act as lateral and bottom seals.7


Fault seal analysis studies three main characteristics:

• Juxtaposition.

• Fault-zone deformation process.

• Reactivation.

Juxtaposition identifies lithological boundaries for foot and hanging walls along the fault.9 Juxtaposition diagrams calculate each fault's small throws consisting of formation interactions (Fig. 3).10 These diagrams can provide fault-seal assessments without the need for 3D images of faults and formations. They can also further identify contour sealing capacity of a particular fault.

Juxtaposition can find critical fault throws helpful in delineating zones with hydrocarbon reservoirs. Juxtaposition analysis determines whether fault seals are the result of lithological differences or the faulted rock itself.

Allan diagrams offer a simpler method of predicting fault seals by calculating complex stratigraphy divisions across a fault (Fig. 4).11 An Allan diagram digitally superimposes the hanging and foot walls and provides a clear representation of a normal fault. The diagram's time scale is the same as that of the seismic sections, and calculation relies on the base map units' scale along the strike.

The fault zone deformation process observes fault zone digenesis, clay smear (shale gouge), grain sliding, and cataclasis.11 These characteristics are important because they can each contribute to fault sealing which includes sand-on-sand juxtaposition. Clay smear indicates that a higher shale-to-sand ratio will incorporate more clay into the fault zone. Predictive algorithms for estimating fault seal potential include shale gouge ratio (SGR) and shale smear factor (SSF).

SGR represents the percentage of shale or clay material that has slipped past a point on the fault. Thicker source clay beds produce more clay smears. Shear-type smears decrease in thickness with distance from clay source layers. Researchers have calculated SGR both for reservoirs with discrete shale-clay beds and for shaly sand reservoirs with discrete shale beds.12

SGR values determine the difference between sealing and non-sealing faults with ratios of 18% or greater corresponding to important fault-seal barriers. A lack of well data has prevented calculation of the pressure difference on either side of the fault plane for the study presented in this article.

Broad evidence from subsurface studies shows that fault zones with an SGR > 20% act increasingly as seals for oil, and outcrop observations confirm that continuous clay smears on fault planes occur where SGR > 15%.12 More recent studies demonstrate that, with higher net-to-gross ratios, a cut-off SGR > 25% increases confidence in predicting fault seal.

Calculating SSF ratios can determine the smear sealing effect.13 SSF remains the same between offsets because it's independent of smear distance. But laterally SSF may be affected by fault throw. SSF representations of abrasion and shale smears may therefore become useless for SSF > 7%.14 Lower values will have more chance to smear, increasing the sealing effect. This ratio is not applicable for complex smears because thin shale beds are capable of producing high SSF values.

Sinjhoro study

Fault and horizon maps in the study area identify several prospects in the post-rift sequence (Fig. 2). Assessing the fault-seal risk is important as most of these play types are fault closures.15

This study analyzed Vshale curves from four previously drilled wells to determine SGR. Fault-seal lithology analysis is derived from reflector continuity in available 2D seismic and well data. There are many faults associated with other leads in the Lower Goru formation, but our study analyzed seals on two specific faults-F1 and F2.

Most of the faults at the top of the Lower Goru formation developed during Lower Cretaceous through Early Paleocene. SGR is calculated and analyzed in a triangle juxtaposition diagram, but most faults contain a sheared mixture in which the fault offset is greater than the bed thickness.16 For this reason, SGR is mainly studied in the interval where fault throw is greater than bed thickness. For simpler calculation, fault zones are considered as single faults through generalized classification based on SGR and SSF values.12

Primary faults

Faults F1 and F2 are normal faults (Lower Cretaceous to Paleocene) at the top of the Lower Goru formation in the central part of the study area. Fault strikes trend NE-SW with a maximum throw of about 100 m at the reservoir top. Existing well data-CHAK66-01 close to F1 and CHAK63-01 near F2-are incorporated in the fault-seal analysis.

At this location, the Lower Goru formation is overlain by a thick sequence of Upper Goru formation with basal sand and Talar shale below the reservoir. Juxtaposition of the Upper and Lower Goru, basal sand, and Talar shale is applied at 1,770-3,000 m after calculating the fault throws and depths along the fault's strike (Fig. 3).

Fault E shows both the juxtaposition of various footwall lithologies against the hanging wall lithology as seen in Fig. 3 and the throw variation along the fault's length. The fault has been extrapolated near to its origination point. In the figure, AA represents self-juxtaposition of Upper Goru in the footwall against the hanging wall. AB represents the juxtaposition of Lower Goru against the Upper Goru in the footwall. BC represents juxtaposition of Lower Goru in the footwall against Talar shale in the hanging wall.

Interpretation of this diagram provides information about the juxtaposition of various units in the footwall against the hanging wall. Sand and shale intervals are determined by the Vshale curve's cutoff value. Seismic data identifies a maximum 100-m throw for the sand reservoir, which gradually increases in the studied section.

The Allan diagram indicates that the Upper Goru formation's Talar shale bound the basal sand and Lower Goru formation, which may provide effective sealing (Fig. 4). Our study calculated SGR and SSF at each seismic line near the top of the reservoir to identify fault-rock properties (Figs. 5 and 6). Juxtaposition indicates an SGR greater than 50% between 30-75 m of throw and SSF less than 5%. This fault will act as a sealing fault because no sand-on-sand juxtaposition lies within the throw interval and the SGR is greater than 50%.

Effective seals

Fault seal analysis concludes that the majority of the formations in the study serve as efficient traps for hydrocarbon on the hanging wall with prospective reservoirs on the foot wall. Fault categorization is based on wall rock juxtapositions, SGR, and SSF. Detailed analysis of both F1 and F2 faults provided a fault communication map showing the top of the reservoir (Fig. 7). Portions within each fault are differentiated by sealing properties.

Faults in the study area are generally classified as sealing unlikely, poor seal, moderate seal, and likely sealed. F1 and F2 are near perfect seals at the identified leads but sealing decreases with more throw.


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The Authors
Gulraiz Akhter ([email protected]) is a professor at Quaid-i-Azam University (QAU), Islamabad. He has also served as chairman of QAU's Department of Earth Sciences. He holds a masters in geophysics (1984) and a PhD in hydrogeology from QAU. He is a member of the Pakistan Geophysical Network (PGN).
Bilal Aslam [[email protected]] is the assistant manager of geophysics with Pakistan's National Space Agency. He holds master of philosophy in geophysics (2014) from QAU, Islamabad. He is a member of PGN and the Pakistan Association of Petroleum Geoscientist (PAPG).
Anees Ahmad Bangash ([email protected]) is an assistant professor in the Department of Earth Sciences with QAU, Islamabad. He holds a BS (1988) in physics, an MS (1991) and MPhil (1995) in geophysics. He is a member of PGN.