Spectral remote sensing for onshore seepage characterization: A critical overview

Abstract

In this article, we overview the application of spectral remote sensing data collected by multi-, and hyperspectral instruments in the visible-near infrared (VNIR), short-wave infrared (SWIR), and longwave infrared (LWIR) wavelengths for characterization of seepage systems as an exploration indicator of subsurface hydrocarbon (HC) accumulations. Two seepage systems namely macro-, and microseepage are recognized. A macroseepage is defined as visible indications of oil and gas on the surface and in the air detectable directly by a remote sensing approach. A microseepage is defined as invisible traces of light HCs in soils and sediments that are detectable by its secondary footprints in the strata, hence an indirect remote sensing target. Based on these broad categories, firstly, a comprehensive set of well-described and reliable remote sensing case studies available in the literature are thoroughly reviewed and then systematically assessed as regards the methodological shortcomings and scantiness in data gathering, processing, and interpretation. The work subsequently attempts to go through seminal papers published on microseepage concept and interrelated geochemical and geophysical techniques, exhumed HC reservoirs, lab-based spectroscopic analysis of petroleum and other related disciplines from a remote sensing standpoint. The aim is to enrich the discussion and highlight the still unexplored capabilities of this technique in accomplishing exploration objectives using the concept of seepage system. Aspects of seepage phenomenon in environmental pollution and uncertainties associated with their role in global warming are also underlined. This work benefits from illustrative products generated over two study areas located in the Ventura Basin, State of California, USA and the Tucano Basin, State of Bahia, Brazil known to host distinctive macro-, and microseepage systems, respectively. In conclusion, we recommend further research over a diverse range of seepage systems and advocate for a mature conceptual model for microseepage phenomenon.

Introduction

A large portion of hydrocarbon (HC) traps is not perfectly sealed and thus, their accumulations leak to the surface over time. When the surface manifestation of oil and gas is clearly visible by naked eye, it is termed as macroseepage, whereas the traces of invisible light HCs in near-surface soils and sedimentary rocks (sediments henceforth), which are only detectable by analytical methods and careful geochemical sampling, is called microseepage (Horvitz, 1985, Tedesco, 1995). Historically, seepage and HC accumulations have been tied together and, as a result, a large number of the world's oil and gas fields have been explored by drilling in the immediate area of a seep (Hunt, 1996, Yergin, 1992). In modern exploration programs, macroseeps are typically regarded as direct clues for the existence of mature source rock(s) and a compelling evidence for the formation of a petroleum system in a given sedimentary basin (Magoon and Beaumont, 1999, Schumacher, 2010), whereas microseeps, which are argued to occur in a near vertical fashion over accumulations, are employed as a targeting tool for petroleum exploration.

Recent investigations have also revealed that seeps are a potent source of methane (with ethane and propane) greenhouse gasses to the atmosphere. It has been estimated that in the natural methane budget, seeps are the second most important source of emissions after wetlands. The estimates also reveal that onshore seepages are a more significant emitter of CH₄ than their offshore counterparts (Etiope, 2015, Etiope and Ciccioli, 2009, Etiope and Klusman, 2010, Etiope et al., 2008).

Over the years, a diverse range of techniques, including remote sensing, has been employed for detecting the indications of seepage systems. The remote sensing approach holds a great promise for this aim because it is a fast and cost-effective tool that can be applied to different operational scales for both direct and indirect seepage mapping. In the marine environment, this technology already provides a variety of sensing methods comprising laser florescence, synthetic aperture radar (SAR), and thermal infrared, to name a few (Leifer et al., 2012a). Terrestrial seepage detection, however, has relied heavily on spectral data collected by multi-, and hyperspectral instruments in the visible-near infrared (VNIR; 0.4–1.0 μm), the short-wave infrared (SWIR; 1.0–2.5 μm), and very occasionally in the longwave infrared (LWIR; 8–14 μm) wavelengths. This methodology has been employed to detect oil and gas seeps in a direct manner and the footprints of HC leakage in sediments indirectly.

Whereas direct detection of HCs is a new research topic conducted mainly by airborne imaging spectroscopy, indirect HC prospecting commenced with the launch of the first Landsat multispectral scanner (ERTS) nearly four decades ago (Simpson, 1978). The advent of the hazy anomaly over some productive/prospective fields derived from Landsat imagery was simultaneous to a renewed interest in microseepage concept (Donovan, 1974) and thereupon led onto several research studies, including a NASA-Geosat joint project, to evaluate the use of satellite technology for HC prospecting (Abrams et al., 1985). Since then, sporadic case studies have appeared in the literature demonstrating the potentials and premises of the approach for oil and gas exploration (see van Der Meer et al. (2002) and Yang et al. (2000) and references therein), albeit in comparison to extensively studied offshore cases are almost negligible (Fingas and Brown, 2014, Leifer et al., 2012a).

Despite all the merits of spectral remote sensing, the technique does not yet hold a good place among unconventional exploration methods for oil and gas resources nor does it acknowledged as a mature exploration tool by explorationists. In this article, we set out to discover the full potential of this state-of-the-art technology for seepage characterization and understand the reasons for which the approach is underutilized by the petroleum industry. To fulfill this aim, we provide a systematic and critical overview of the subject based on well-described and reliable remote sensing case studies reported in the literature and thereupon attempt to address methodological shortcomings and inadequacies in data gathering, processing, and interpretations. In addition, we go through the seminal papers published on microseepage theory and interrelated geochemical and geophysical techniques from a remote sensing standpoint to enrich the discussion and highlight the unexplored capabilities of the technique in accomplishing the objectives of the exploration sector. We also attempt to engage the attention of the community to useful case studies conducted over exhumed HC reservoirs as an analogy to depleted reservoirs. In the case of direct seepage detection, we review the few present case studies and contrast them with the findings of interrelated disciplines to underline a wide range of possibilities from spectral products. Lastly, we accentuate the uncertainties about the role of natural seepage in greenhouse gas emission and the possible strategies to reduce it. This paper benefits from illustrative products generated over two case studies located in the Ventura Basin, California, USA and the Tucano Basin in the Bahia state, Brazil known to host distinctive macro-, and microseepage systems, respectively.