Conceptual modeling of onshore hydrocarbon seep occurrence in the Dezful Embayment, SW Iran

doi:10.1016/j.marpetgeo.2013.03.001

Marine and Petroleum Geology

Volume 43, May 2013, Pages 102-120

https://doi.org/10.1016/j.marpetgeo.2013.03.001 Get rights and content

Abstract

Petroleum and gas seeps on the ground surface are direct indicators of accumulations of hydrocarbons in the subsurface and could reflect the migration of hydrocarbons in a sedimentary basin. Quantitative analyses of the spatial pattern of hydrocarbon seeps and their spatial associations with geological features could aid in deducing geological controls on their occurrence. In this study the Fry analysis was applied to study the spatial pattern of mapped hydrocarbon seeps, whereas spatial association analyses were implemented to quantify the spatial association of mapped seeps and their alteration products with geological features. The spatial pattern analysis of hydrocarbon seeps showed that oil seeps followed prominent NW–SE and NE–SW trends while gas seeps followed NW–SE and N–S trends suggesting that NNE–SSW and NW–SE fractures are possible migration pathways for hydrocarbons to reach the surface. The results of the spatial association analysis illustrated strong positive spatial associations of oil and gas seeps with the Gachsaran and the Mishan formations, implying upward migration of hydrocarbons through permeable micro-fractures and micro-pores in their strata. A conceptual model has proposed for the occurrence of onshore hydrocarbon seeps in the Dezful Embayment.

Highlights

► The spatial pattern of hydrocarbon seeps is described in the Dezful Embayment. ► Spatial associations of hydrocarbon seeps with geological features have been analyzed. ► Quantitative analyses aid in deducing geological controls on hydrocarbon seeps occurrence. ► A conceptual model has proposed for the occurrence of seeps in the Dezful Embayment.

Introduction

Petroleum and gas seeps on the ground surface are direct indicators of accumulations of hydrocarbons in the subsurface and could reflect the migration of hydrocarbons in a sedimentary basin. Due to high pressures at depths, hydrocarbons in the subsurface can escape to the surface through fractures in rocks and planes of weakness between geological layers.

There are extensive studies about spatial patterns of hydrocarbon seeps in offshore areas (De Boever et al., 2009; Huang et al., 2009; Jin et al., 2011; Washburn et al., 2005) and various conceptual models of hydrocarbon migration in such areas have been proposed (Ding et al., 2008; Leifer and Boles, 2005, 2006). In contrast, few studies have been published about the geological context of hydrocarbon seeps in onshore areas (Clarke and Cleverly, 1991; Link, 1952; MacGregor, 1993). Hydrocarbon seeps are spatially associated with structures such as faults, fractures, folds, unconformities, and salt domes. They can be found within the reservoirs and cap rock formations exposed at the surface. Spatial associations of hydrocarbon seeps with geological features could aid in investigation of cap rock capacity at regional scales (O'Brien et al., 2005; Pinet et al., 2008). In addition, recognition of spatial associations between hydrocarbon seeps with geological features could provide additional valuable information for exploration and environmental programs (Ellis et al., 2001; Etiope et al., 2006).

Methods for quantitative analyses of the spatial pattern of mineral deposits and their spatial associations with geological features have been extensively applied for mineral prospectivity mapping (Carranza, 2009a, Carranza, 2009b; Carranza and Sadeghi, 2010). Such methods have not yet been applied, however, to study hydrocarbon seep occurrences. Like in mineral prospectivity analysis, quantitative analyses of the spatial pattern of hydrocarbon seeps and their spatial associations with geological features such as faults, fractures, and lithologies could aid in deducing geological controls on their occurrence. Analysis of the spatial pattern of mapped hydrocarbon seeps could provide insights into which geological features control their localization at the surface. In addition, analysis of spatial association of mapped seeps with geological features is instructive in weighing the importance of each geological feature as controls on the occurrence of hydrocarbon seeps. Consequently, such analyses would allow development of a conceptual model of how hydrocarbon seeps are localized at the surface and why they occur only at specific sites.

In this paper, we describe quantitatively the spatial pattern of mapped hydrocarbon seeps in the Dezful Embayment, SW Iran. The prime objectives of this study are to obtain insights about (a) links between geological structures such as faults, fold axes and fractures and hydrocarbon seeps and (b) the probability of spatial association of different geological features such as lithological units and their associated structures with oil and gas seeps. We chose the Dezful Embayment for this study because sufficient knowledge exists about the distribution of petroleum at the surface and in the subsurface.

Section snippets

Geological setting of the study area

The study area is located in the Zagros fold-thrust belt, SW Iran. The co-existence of rich source rocks, excellent reservoirs, efficient seal, large anticlines and rock fractures resulting from the Zagros folding explain the importance of the Zagros fold-thrust belt as a prolific petroleum province (Bordenave and Hegre, 2005). At least 40–50% of Iranian seeps in the SEEPS database, which was produced by the British Petroleum and described by Clarke and Cleverly (1991), can be linked to

Methods

We used 14.1:100,000 scale geological maps of the Dezful Embayment, which have been compiled by the Iranian oil company, to provide lithologic, structural, and hydrocarbon seep maps. Existing knowledge about petroleum systems in the Zagros oil fields (Bordenave, 2002; Bordenave and Hegre, 2010), and extensive studies about the structural framework of the Zagros fold-thrust belt (Berberian, 1995; Bordenave and Hegre, 2005; Carruba et al., 2006; Hessami et al., 2001; McQuillan, 1991; Sepehr and

Spatial patterns of hydrocarbon seeps

Mapped hydrocarbon seeps in the Dezful Embayment show prominent 315°± 15 trend (Fig. 5), which is roughly the same as the general trend of folds and faults in the Zagros fold-thrust belt. Fry plots of hydrocarbon seeps at local scales (<43 km) show subsidiary NNE–SSW trend (Fig. 5a). At the local scale, the Fry plots of heavy oil or asphalt seeps show a prominent NW–SE trend and a subsidiary NNE–SSW trend (Fig. 6). The Fry plots of Gach-e-tursh and sulphur springs show prominent NW–SE and

Conceptual modeling of occurrence of hydrocarbon seeps in the Dezful Embayment

The strong positive spatial associations of oil seeps and products of gas seeps alteration such as sulphur springs and Gach-e-tursh with the Mishan Formation and with anticline axes and thrust faults in that formation (Table 2) imply that densities of hydrocarbon seeps are not decreased upwards in the stratigraphy. The results of spatial association analyses (Figs. 9 and 10) imply that there are probably two groups of heavy oil or asphalt seeps. The first group is spatially associated with the

Discussion

The results of Fry analyses of hydrocarbon seep locations are useful for visual interpretations of which structures likely controlled the occurrence of hydrocarbon seeps in the Dezful Embayment. The syntheses of those results with the results of analyses of spatial association of seeps with geological features are useful for development of a conceptual model for the occurrence of onshore hydrocarbon seeps in the Dezful Embayment. In this conceptual model, we suggest that hydrocarbon seeps and

Concluding remarks

With the aid of spatial pattern and spatial association analyses, a conceptual model for the occurrence of oil and gas seeps in the Dezful Embayment was developed. Oil seeps likely have migrated along NW–SE and NE–SW trending faults and fractures in SW flanks of anticlines, whereas gas seeps likely have escaped to the surface through prominent NW–SE and N–S trending faults and fractures. The positive spatial associations of seeps with the Mishan Formation imply that the density of seeps is not

Acknowledgment

The authors thank the European Union – in particular the Erasmus Mundus programmme – for financial support of the research.

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Geology and hydrochemistry of brine springs in the Zagros Fold and Thrust Belt (ZFTB), Iran: A review on origin, environmental aspects, and economic potentials
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This passive oil seepage suggests the existence of a petroleum system in both basins. The discovery of many of the world's great oil fields, such as the eastern foothills of the Sierra de Perijá, the Lake Maracaibo Basin (Venezuela), the Dezful Embayment (SW Iran), and the Bagua Basin (north-central Peru) were a direct consequence of seepage drilling (Escobar et al., 2011; Salati et al., 2013; Gentzis, 2013). Samaila et al. (2008) have previously documented the occurrence of microstructures within the Bima Formation.
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¹: Present address: School of Earth and Environmental Sciences, James Cook University, Townsville, Queensland 4811, Australia.

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Conceptual modeling of onshore hydrocarbon seep occurrence in the Dezful Embayment, SW Iran

Abstract

Highlights

Introduction

Section snippets

Geological setting of the study area

Methods

Spatial patterns of hydrocarbon seeps

Conceptual modeling of occurrence of hydrocarbon seeps in the Dezful Embayment

Discussion

Concluding remarks

Acknowledgment

Marine and Petroleum Geology

Ore Geology Reviews

Ore Geology Reviews

Marine and Petroleum Geology

Tectonophysics

Marine and Petroleum Geology

Marine and Petroleum Geology

Marine and Petroleum Geology

Marine and Petroleum Geology

Marine and Petroleum Geology

Journal of Southeast Asian Earth Sciences

Marine and Petroleum Geology

Marine and Petroleum Geology

Marine and Petroleum Geology

Marine and Petroleum Geology

Neogene salt in SW Iran and its interaction with Zagros folding

Geological Magazin

Regional stratigraphy of the Zagros fold-thrust belt of Iran and its proforland evolution

American Journal of Science

Sedimentary Basins and Petroleum Geology of the Middle East

Structural variation along the Zagros and the nature of the Dezful Embayment

Geological Magazine

Master “blind” thrust faults hidden under the Zagros folds: active basement tectonics and surface morphotectonics

Tectonophysics

Towards a paleogeography and tectonic evolution of Iran

Cnadian Journal of Earth Sciences

Distance distributions associated with Poisson processes of geometric figures

Journal of Applied Probability

Petroleum in the Zagros Basin: a late Tertiary foreland basin overprinted onto the outer edge of the vast hydrocarbon-rich Paleozoic–Mesozoic passive margin shelf

Geographic Information Systems for Geoscientists: Modelling with GIS

The Middle Cretaceous to Early Miocene Petroleum System in the Zagros Domain of Iran, and Its Prospect Evaluation

The influence of tectonics on the entrapment of oil in the Dezful Embayment, Zagros foldbelt, Iran

Journal of Petroleum Geology

Current distribution of oil and gas field in the Zagros fold belt of Iran and contiguous offshore as the result of the petroleum systems

Geological Society of London