China Science

Cretaceous-Paleogene overpressure distribution in the Danish Central Graben shows a remarkable coincidence with the thickness of the rapidly deposited middle Miocene to Holocene succession.Slow deposition of smectite-dominated clays in a deep-marine environment occurred from the late Paleocene until the middle Miocene,and the resultant mudstone succession constitutes the main barrier that delays pressure dissipation.Between the late Miocene and the Holocene,the Upper Cretaceous-Paleogene succession became over-pressured,probably because of accelerated depositional rates.Quantification of this disequilibrium compaction mechanism relies mainly on a determination of permeability and effective compressibility of the Paleogene shales.This article shows that realistic permeabilities can be assumed,provided that compressibilities describing the plastic process of compaction are used in the pressure equation instead of the elastic compressibilites that,for instance,can be derived from log data.One-dimensional(1-D)modeling is applied in two cases:a well from the Dan chalk field,where accelerated deposition since the Tortonian(11.2 Ma)produced a present-day overpressure of 7.97 MPa(1156 psi);and a well from the South Arne chalk field,where accelerated deposition since the early Serravallian(14.6 Ma)produced a present-day overpressure of 13.9 MPa(2016 psi).This is based on an identical set of parameters and compares with the observed 7.7 and 14.8 MPa(1117 and 2147 psi)overpressure at the two locations.The modeled development of the pressure profiles shows that an effective stress minimum occurred in the upper part of the Paleogene succession.This is consistent with the observed ubiquitous intraformational faulting at that level.About 80% of the added Neogene load is estimated to have been converted to overpressure.

Ole Valdemar Vejbaek

A publicly available data set has been examined for relationships between average values of porosity,permeability,depth,temperature,pressure,thickness,age,and play type for 11,833 sandstone reservoirs,mostly of Miocene age and younger,from the United States offshore Gulf of Mexico(GOM).Porosity shows wide scatter as a function of burial depth,but the median(P50)porosity trend decreases smoothly with depth.The GOM trend has much higher porosity for the given depth than the P50 trend of sandstone reservoirs worldwide,reflecting rapid sedimentation rates and young ages of GOM reservoirs,most of which have spent relatively little time at temperatures more than 80°C,where quartz cementation becomes active.Multivariate regression analysis shows that porosity is best predicted by temperature(r2=0.40),with the fit improved slightly by adding age and then depth(r2=0.44).Arithmetic average permeability(represented by its logarithm)shows a correlation of maximum and P50 trends with porosity.GOM P50 permeability lies 0.2-0.4 log units below the P50 trend for sandstone reservoirs worldwide,probably reflecting very fine grain size of most GOM sands.Water saturation can be used to calculate the effective(petroleum-filled)porosity of each reservoir,which shows strong correlation with permeability.Grouping the reservoirs by chronozone reveals regular trends of decreasing average porosity and permeability with increasing age,reflecting increasing average depth and temperature with age.Porosity and permeability functions representing depositional sand quality show only subtle differences between different age groupings and play types.The results presented here can be useful for specifying realistic distributions of parameters for both exploration risk evaluation and reservoir modeling.

S.N.Ehrenberg?P.H.Nadeau?and O.Steen

James C. Hollingsworth, a member of AAPG for over 50 years,passed away on September 26, 2007, at the age of 84. He was born to Clarence Wise and Zephyr Bryan Hollingsworth of Gahagan, Louisiana on July 31, 1923. He was proud to serve his country in the United States Army from 1943 to 1946 in radio intelligence. He served overseas during WWII including landing on the beaches of Normandy on D-Day. For meritorious achievement in connection with military operations from December 1944 to May 1945 in France and Germany, he was awarded the Bronze Star Medal. He was recognized for his devotion to duty, his fine personal characteristics, and tireless energy-all of which he taught to his three children. Upon his discharge and return to Shreveport, Louisiana, he attended Centenary College where he received his Bachelor of Science in geology, Magna Cum Laude. He then attended Colorado School of Mines and received his Master of Science degree. He earned a membership in the honor fraternity Omicron Delta Kappa and also received honors in Sigma Gamma and Alpha Sigma Pi.

Cheryl Denise Hollingsworth

Dr. John D. Cooper, Professor Emeritus at California State University, Fullerton,and a driving force in the operation of the Pacific Section, SEPM, died of a heart attack on September 3, 2007 while on a morning hike near his home in Chino Hills, California. His pleasant personality, enthusiasm, creative ideas, and contributions to the field of sedimentary geology will be greatly missed. John was born on June 12, 1939 in Wichita, Kansas. His childhood was spent in Blacksburg, Virginia where his father taught geology at Virginia Tech. He learned outdoor skills and a love of the outdoors both from his father and from being in the Boy Scouts, where he achieved the rank of Eagle Scout. He earned his bachelor’s degree in Geology from the University of Michigan in 1961, and then moved on to the University of Texas where he finished his master’s degree and, in 1970, his Ph.D. in geology.

A. Eugene Fritsche

Normal faults in Cretaceous carbonates in the Balcones fault system provide important analogs for fault zone architecture and deformation in carbonate reservoirs worldwide. Mechanical layering is a fundamental control on carbonate fault zones. Relatively planar faults with low-displacement gradients develop in massive, strong, clay-poor limestones and dolomites. In less competent clay-rich strata, shale beds impede fault propagation, resulting in fault-related folding, and locally steep bedding dips. Faults in clay-poor massive limestones and dolomites tend to be steep (70° or more), whereas weaker, clay-rich limestones develop faults with shallower dips (60° or less). Fault zone rocks show evidence of cataclasis, cementation, deformation of cement by mechanical twinning and pressure solution, and multiple generations of cement with differing degrees of deformation, indicating contemporaneous cementation and fault slip. In stratigraphic sequences consisting of both competent and incompetent strata, the ratio of incompetent to competent strata by thickness is a useful guide for inferring the relative rates of fault displacement and propagation. Low displacement-to-propagation ratios associated with competent strata generate low-displacement gradients, inhibiting fault-related folding. Conversely, high displacement-to-propagation ratios associated with incompetent strata promote high-displacement gradients and fault-related folding.

David A. Ferrill?Alan P. Morris
Geosciences and Engineering Division, Department of Earth, Material, and Planetary Sciences, Southwest Research Institute~R, 6220 Culebra Road, San Antonio, Texas 78238

Differentiation of microbial versus thermogenic methane in coalbed and black shale accumulations can affect strategies for exploration and may influence the total gas content in a given area. Early identification of these processes from crushed core materials, even before formation fluids and produced gas samples are available, could permit a more efficient and cost-effective exploration. Total gas contents and compositional and isotopic data from New Albany Shale core materials are presented, which delineate regional occurrence of microbial, thermogenic, and mixed gas generation in the Illinois Basin. These trends are consistent with those identified from detailed prior studies of produced gas and water chemistry from the same locations. The most useful markers for microbial gas in crushed core gases are elevated CO2 contents characterized by high ?~(13)C_(CO2) values (> 5 per thousand). Core gas analyses from wells in which microbial gas is identified commonly have significantly more total gas absorbed than do core samples from wells producing gases solely of thermogenic origin. These observations are independent of variations in sample depth and organic carbon content in a given core. Thus, this integrated case study of core and produced gases in the Illinois Basin illustrates that the areas containing microbial gas, in addition to early thermogenic gas, may be more productive than pure thermogenic zones for these early to immature unconventional gas deposits.

Anna M. Martini?Lynn M. Walter?Jennifer C. Mcintosh
Department of Geology, Amherst College, Amherst, Massachusetts 01002; Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109; Department of Hydrology and Water Resources, University of Arizona, Tucson, Arizona 85716

We present a generic algorithm for automating sedimentary basin reconstruction. Automation is achieved through the coupling of a two-dimensional thermotectonostratigraphic forward model to an inverse scheme that updates the model parameters until the input stratigraphy is fitted to a desired accuracy. The forward model solves for lithospheric thinning, flexural isostasy, sediment deposition, and transient heat flow. The inverse model updates the crustal- and mantle-thinning factors and paleowater depth. Both models combined allow for automated forward modeling of the structural and thermal evolution of extensional sedimentary basins. The potential and robustness of this method is demonstrated through a reconstruction case study of the northern Viking Graben in the North Sea. This reconstruction fits present stratigraphy, borehole temperatures, vitrinite reflectance data, and paleowater depth. The predictive power of the model is illustrated through the successful identification of possible targets along the transect, where the principal source rocks are in the oil and gas windows. These locations coincide well with known oil and gas occurrences. The key benefits of the presented algorithm are as follows: [1] only standard input data are required, (2) crustal- and mantle-thinning factors and paleowater depth are automatically computed, and (3] sedimentary basin reconstruction is greatly facilitated and can thus be more easily integrated into basin analysis and exploration risk assessment.

L. H. Rupke?S. M. Schmalholz?D. W. Schmid
Physics of Geological Processes, Oslo University, P.O. Box 1048 Blindern, 0316 Oslo, Norway; Geological Institute, Leonhardstrasse 19, ETH Zurich, 8092 Zurich, Switzerland

The Mesozoic (Upper Jurassic-Lower Cretaceous) deeply buried gas reservoir play in the central and eastern Gulf coastal plain of the United States has high potential for significant gas resources. Sequence-stratigraphic study, petroleum system analysis, and resource assessment were used to characterize this developing play and to identify areas in the North Louisiana and Mississippi Interior salt basins with potential for deeply buried gas reservoirs. These reservoir facies accumulated in Upper Jurassic to Lower Cretaceous Norphlet, Haynesville, Cotton Valley, and Hosston continental, coastal, and marine siliciclastic environments and Smackover and Sligo nearshore marine shelf, ramp, and reef carbonate environments. These Mesozoic strata are associated with transgressive and regressive systems tracts. In the North Louisiana salt basin, the estimate of secondary, nonassociated thermogenic gas generated from thermal cracking of oil to gas in the Upper Jurassic Smackover source rocks from depths below 3658 m (12,000 ft) is 4800 tcf of gas as determined using software applications. Assuming a gas expulsion, migration, and trapping efficiency of 2-3%, 96-144 tcf of gas is potentially available in this basin. With some 29 tcf of gas being produced from the North Louisiana salt basin, 67-115 tcf of in-place gas remains. Assuming a gas recovery factor of 65%, 44-75 tcf of gas is potentially recoverable. The expelled thermogenic gas migrated laterally and vertically from the southern part of this basin to the updip northern part into shallower reservoirs to depths of up to 610 m (2000 ft).

Ernest A. Mancini?Peng Li?Donald A. Goddard
Center for Sedimentary Basin Studies and Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487; Arkansas Geological Survey, Little Rock, Arkansas 72204; Center for Energy Studies, Louisiana State University, Energy, Coast & Environment Building, Baton Rouge, Louisiana 70803