Distribution Of Gas Accumulations Research Articles

THE ORIGIN OF THE PERMO‐TRIASSIC GAS ACCUMULATIONS IN THE IRANIAN ZAGROS FOLDBELT AND CONTIGUOUS OFFSHORE AREAS: A REVIEW OF THE PALAEOZOIC PETROLEUM SYSTEM

More than 1500 trillion cubic feet (Tcf) of gas reserves have been discovered in Permo‐Triassic carbonates sealed by thick Triassic anhydrites in the Zagros Foldbelt (SW Iran), the southern part of the Gulf (Iran, Qatar, and Abu Dhabi) and Saudi Arabia. This paper discusses the origins of this gas in terms of source rock distribution and thermal maturation through time (as indicated by modelling), regional variations in thermal maturation (as indicated by cumulative isopachs), and long‐range migration and accumulation of hydrocarbon prior to the Zagros orogeny. The sequence of events leading to the present‐day distribution of gas accumulations is reconstructed in detail.The only important source rocks so far identified in the Late Proterozoic to Late Triassic succession are organic‐rich, radioactive shales which are dated as Llandoverian (early Silurian). Oil generation began in the Middle Jurassic in areas of greatest subsidence, while the gas window was reached locally as early as the Middle Cretaceous. Huge volumes of oil, then of gas, accumulated in a few major regional highs and salt‐related structures prior to the Zagros orogeny. Part of the gas was lost during Zagros folding as some of the anticlines were breached, and another portion, possibly associated with light oil, remigrated into unbreached Zagros anticlines.Among critical parameters essential to the appraisal of the numerous Permo‐Triassic prospects present in Lurestan, Fars and in the Iranian Offshore, three are discussed in this paper, namely (i) the location of prospects in comparison to pre‐Zagros regional highs and to reconstructed pre‐Zagros gas accumulations; (ii) the characteristics of potential reservoir intervals in the Dalan/Kangan Formation; and (iii) the extent of the Dashtak evaporitic seal. The distribution of surface oil, bitumen and gas seepages together with indirect hydrocarbon indications provides an additional exploration tool.

Geochemistry and origin of sour gas accumulations in the northeastern Sichuan Basin, SW China

Significant natural gas reserves have recently been discovered in the Lower Triassic oolitic reservoirs from northeastern Sichuan Basin, SW China. In the wake of the December 2003 sour gas well blow-out, this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area. Two types of natural gas accumulations were identified in the Lower Triassic oolitic reservoirs, both containing highly mature thermogenic gases, with their hydrocarbon source rocks in Upper Permian strata. Natural gases from the area south of the ancient Kaijiang-Liangping Seaway are generally sweet gases formed as the result of thermal maturation, whereas those discovered from north of the Seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the Feixianguan Formation reservoirs. The proposed origins of the gases are supported by their chemical and stable carbon isotope compositions, as well as the presence or absence of pyrobitumens in the reservoir. The distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors. The regional variation in the concentrations of H 2S in the gases appears related to the presence and thickness of anhydrite-bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs.

Simulating the Spatial Distribution of Undiscovered Petroleum Accumulations

Abstract There are two major problems in the construction of a stochastic model that describes quantitatively the spatial distribution of undiscovered petroleum accumulations: a) available exploration results are biased; and, b) information associated with the locations of accumulations is incomplete. Studies in the Western Canada Sedimentary Basin (WCSB) and elsewhere indicate that the spatial characteristics of petroleum accumulations are fractal. In this paper, we propose the use of these fractal characteristics to calibrate sampling bias, thus deriving an unbiased spatial correlation (covariance function) for the stochastic modelling. The uncertainty in the modelled locations of undiscovered accumulations resulting from insufficient information is captured by equal-probable realizations of the simulation and these are subsequently converted to a probability map of petroleum occurrence. In the example, a pre-1994 exploratory data set for the Rainbow gas play in WCSB was used to derive simulation parameters. A comparison of the simulated results to post-1993 gas discoveries in the same play shows that most of the post-1993 discoveries are located in areas with high predicted probability values. Introduction Spatial characteristics of undiscovered oil and gas accumulations are important for both better natural resource management and improved exploration efficiency. There are two major obstacles associated with the construction of a stochastic model for describing the spatial distribution of undiscovered petroleum accumulations. The first is that the available information is biased with respect to exploration results. The second is that information associated with the locations of petroleum accumulations is incomplete unless all accumulations are discovered. It is well known that the data associated with the discovery of petroleum accumulations in an exploration program is biased. Larger features are generally tested with higher priority(1,2). This sampling bias prohibits the use of conventional methods to estimate stochastic model parameters. Barton et al.(3) and La Pointe(4) studied the data from well-explored petroleum basins in the United States and concluded that the spatial distribution of hydrocarbon accumulations is fractal. Our studies in the WCSB also indicate that the spatial distribution of petroleum accumulations exhibits a similar characteristic. Figure 1 shows the box counting results for the Rainbow gas play in the WCSB. The linear relation between box size and the number of boxes containing gas pools (on a logarithmic scale) indicates fractal geometry of the spatial distribution of gas accumulations. The scaling property of the spatial objects means that spatial characteristics of large objects of petroleum accumulations could be used to infer the spatial characteristics for the smaller ones, which are underrepresented in the data set. Thus, an unbiased spatial structure of petroleum accumulations could be inferred from the biased observations. By transforming the spatial information of discovered hydrocarbon accumulations into a frequency domain using a fast Fourier transform (FFT), it results in an amplitude map and a phase map. The amplitude map contains information associated with spatial correlation, while the phase map contains location specific information. When both the location-specific information in the phase map and the spatial correlation in the amplitude map are complete, the true spatial locations of petroleum accumulation can be inferred.