Eocene Coals Research Articles

Cenozoic basin histories determined by fission-track dating of basement granites, South Island, New Zealand

New apatite (56), sphene (5) and zircon (22) fission-track ages from granitoid rocks are combined with K-Ar ages (both new and published) to reveal a thermo-tectonic history of the West Coast, South Island, New Zealand. Although many of the intrusives are thought to have been emplaced in Palaeozoic time, later thermal events are revealed as a result of resetting of the various isotopic clocks. Modal ages of 105, 95 and 75 Ma on biotite (K-Ar), zircon and apatite (fission track), respectively, represent a Mesozoic cooling/uplift of the granites. A few zircon and many apatite ages contain further overprinting, representing later uplift during the Miocene. On the basis of these events, the West Coast can be subdivided into three tectonic regions. Several of the present granitoid mountains were the sites of NE-SW elongate sedimentary basins in the Early Tertiary. A direct correlation can be made between coal rank (percent vitrinite reflectance) of Eocene coals immediately above basement granites and the underlying granite apatite “age”. Granites, with associated overlying coals having a percent vitrinite reflectance of > 0.7, have been buried sufficiently deeply for the apatites to be reset prior to basin uplift in the Miocene. Those coals with a reflectance of generally < 0.55 overlie granites that have retained a Late Mesozoic age. Intermediate or mixed ages (55 Ma) are found where the vitrinite reflectance of coal lies between these two values, although there is some overlap. These are the product of partial annealing, a conclusion supported by confined track-length distributions. Ages on samples on either side of major faults suggest that these faults may have been active both during basin development and Miocene uplift; hence, their sense of movement has reversed with time.

A modification of procedures for petrographic analysis of tertiary Indonesian coals

A study undertaken to characterize the Eocene coals from southeast Kalimantan has shown that standard preparation procedures fail to capture some basic petrographic properties of the coal. Modification of these procedures permits recognition of distinct plant parts and tissues embedded in finer grained matrix components. Plant parts and tissues can be classified on the basis of morphology and degree of degradation. The highest concentration and best preservation of plant parts and tissues occurs in banded coal and is lowest in the non-banded coal. Use of these procedures, which relates megascopic appearance to petrographic character, should allow more precise utilization of the coal.

Thermo-Tectonic History of Taranaki Basin (New Zealand) Using Apatite Fission Track Analysis (AFTA): ABSTRACT

The Taranaki basin, which extends offshore between the north and south island of New Zealand, contains several large gas fields (e.g., Maui field) and smaller oil fields. The Taranaki basin is New Zealand's only productive hydrocarbon basin. The basin trends north-south, is asymmetrical in cross section, and is faulted with up to 7 km of displacement along parts of its eastern margin. Preliminary results from Apatite Fission Track Analysis (AFTA) reveal the timing and magnitude of basin inversion. Four well cross sections from the southern part of the basin have been used. Initially, basin tectonics and sedimentation were associated with extension and the formation of half-grabens that began in the Late Cretaceous with the breakup of Gondwana. However, most of the observed subsidence and sedimentation resulted from mid-Cenozoic rifting throughout western New Zealand. Following the formation of the modern Australia-Pacific plate boundary during the early Miocene, the southern part of the basin, which lies 60 km from the Alpine fault in places, was partially inverted. AFTA parameters (apparent age and length) downhold at the Fresne-1 well show a distinct break in slope at 1,100 m depth (currently at 30/sup 0/C) where the apparent age is 15 Ma for the Latemore » Cretaceous Parkawau Coal Measures. The data indicate that basin inversion began about 15 Ma and was accompanied by the removal of 2-3 km of section. Sedimentation began again in the Taranaki basin during the mid-Pliocene. The source of the gas and gas condensate in the basin is probably the Eocene coal measures. The maturation history of these beds and the overlying reservoirs was modeled using the constraints from AFTA data. Discrepancies exist between estimates of maximum paleotemperature from AFTA results and from vitrinite reflectance.« less

Stratigraphy, palynology and geochemistry of the Lower Eocene coals of Arauco, Chile

The Arauco Sedimentary basin of Chile (37° −37° 45′S, 73° 08′ −73° 41′W) contains the most important reserves of bituminous coals of Chile. These are concentrated in two Formations: Curanilahue (Lower Eocene) and Trihueco (Middle Eocene). Five coal seams from the Lota Member of the Curanilahue Formation have been studied from the stratigraphic, palynologic and geochemical point of view. These coals were formed during a regressive stratigraphic episode; the palynological assemblage indicates that the climatic conditions prevailing at the time were of high humidity and temperature. These coals from a geochemical point of view are characterized by high sulphur (2.02%), ash (11.9%) and Ge (67.5 ppm; 1224.3 ppm in ash).

New Deeper Exploration Frontiers in Bass Basin: ABSTRACT

The Bass basin is the sister basin to the Gippsland. Both are extensional basins in southeast Australia between Tasmania and the mainland, and both occupy areas in excess of 60,000 km2. The Bass and Gippsland basins were resurveyed seismically by the Bureau of Mineral Resources to provide regional correlation and to penetrate seismic energy barriers caused by Eocene coals. Analyses of these recent good quality deep seismic data, existing seismic data, and well information including thermal maturity, source rock richness, porosity, and permeability suggest that the Bass basin is not fully analogous to the Gippsland basin. Though stratigraphy is similar, there is a paucity of significant secondary faulting in Bass basin to act as paths for migration of hydrocarbons to thermally immature levels, as is the case in the Gippsland basin. Consequently, in spite of minor oil and gas in the thermally immature section, the hydrocarbon potential of Bass basin could be substantially limited to thermally mature levels. Reservoir, seal, and source appear to be present in the Bass at thermally mature but rarely drilled levels within the deeper lower Tertiary and Upper Cretaceous lower Eastern View coal measures and in the rifted Lower Cretaceous Otway Group. Hydrocarbon leads at these levels are predicted from structural analysis, with synsedimentary transverse faults that developed during the extensional phase of the basin providing an important trapping component. Such leads are defined at drillable depths by seismic mapping. The Bass basin could thus ultimately emerge as a deeper hydrocarbon province. End_of_Article - Last_Page 317------------

Geochemical Characterization of Rocky Mountain, Northern Great Plains, and Interior Province Coals: ABSTRACT

Statistical summaries of proximate and ultimate analyses, heat of combustion, and content of 36 major, minor, and trace elements were calculated for 37 Eocene, 470 Paleocene, and 419 Cretaceous coal samples from 31 coal fields or areas in the Rocky Mountain and Northern Great Plains coal provinces and for 503 Pennsylvanian coal samples from 14 areas in the Interior coal province. These analyses show that coals within an age group have similar ranges in composition, and that each group has its own distinctive compositional characteristics. Most variability in element content can be related to changes in rank and differences in ash and total sulfur contents. Mean contents of Ca, Mg, Na, Ba, and Sr are related to rank and decrease as apparent coal rank increases from lignite A to high-volatile B bituminous coal. Mean contents of Si, Al, K, Ti, Ga, Li, Sc, Th, V, Y, and Yb increase as the mean ash content increases (correlation coefficients 0.6), suggesting that these elements are present as aluminosilicates, stable oxides, or phosphate mineral phases. Mean contents of Fe, As, Cd, Co, Cu, Mo, Ni, Pb, Sb, and Zn show high correlation with total sulfur. Contents of these elements are low in Paleocene (0.6% sulfur) and Cretaceous (0.7% sulfur) coals, higher in Eocene (1.8% sulfur) coals, and generally highest in Pennsylvanian (3.9% sulfur) coals. The mean contents of B, Be, Cr, F, Hg, Mn, Nb, Se, U and Zr show no direct relationships to changes in rank or ash and total sulfur contents. Decrease in element content with increased rank probably is related to loss o functional groups that act as cation-exchange sites on organic matter. Ash and sulfur contents are dependent on pH-controlled levels of bacterial activity in ancestral peat swamps. End_of_Article - Last_Page 447------------

Patterns of Thick Coal Deposition Across Powder River Basin in Northeastern Wyoming: ABSTRACT

Tertiary coal beds in the Powder River basin are the nation's largest resource of low-sulfur subbituminous coal. In northeastern Wyoming the north-trending depositional axis of the asymmetric, 100-km-wide basin is within 20 km of the west margin. Interlocking lines of cross sections prepared from geophysical logs of oil and gas wells and coal exploratory holes and set to a sea level datum, outline patterns, locations, and trends of thick coal deposition. Coal beds on the east flank of the basin dip west about 1/2° and tend to merge eastward. Five Paleocene (Fort Union) coals merge along the eastern margin to form a north-trending, 35-m-thick deposit known as the Wyodak coal. Near the depositional axis, five or more Eocene (Wasatch) coals merge to form a north-trendin , 60-m-thick deposit known as the Lake deSmet coal. Across the intervening area two or more succeeding coals merge locally on broad depressed subsurfaces to form north-trending belts of thick coal, and succeeding belts of thick coal are offset westward toward the depositional axis. Lines of cross sections oriented north-south roughly parallel to the depositional strike provide stratigraphic control on the coal. Maximum structural relief (about 400 m) of coal beds on the east flank relates to basin subsidence during deposition of coal-bearing rocks when rates of subsidence progressively increased westward. North-trending patterns of thick coal deposition across paleoslopes are associated with connected areas for which optimum paleoenvironments were stable for extended periods of time and ptimum rates of subsidence were maintained. End_of_Article - Last_Page 732------------

Economic Geology of India 1

II. IN a former notice of Prof. Valentine Ball's important work on the “Economic Geology of India,” the sub jects of the gold supply and of that form of carbon known as the diamond, were treated of. In the present notice it is proposed to give a brief account of that more important form of carbon known as coal, as well as to allude to the valuable information given in the chapters on Iron, Salt, and Building-stone. The rocks, which in Peninsular India probably correspond, as regards the time of their formation, to the true carboniferous rocks of Europe, are not coal-bearing, and the oldest coal-measures in the country belong to a period which is well included within the limits of the Upper Palæozoic or Permian, and the Lower Jurassic formations. All the useful coal of the peninsula may conveniently be described I as being of Permio-Triassic age, and, with two exceptions, it may be added, these measures do not occur beyond the limits of the peninsula. In the extra-peninsular area, coal is found in various younger deposits, and there are numerous deposits in Afghanistan, the Punjab, at the foot of the Himalayas, in Assam and Burma, of undoubted Lower Tertiary, Nummulitic, or Eocene coals and lignites; but it is only quite exceptional that such deposits possess any great value (the chief noteworthy exceptions occur in Assam and Burma).