Early Cretaceous Sandstone Research Articles

Frontier exploration in Western and Northwest Europe

Extended Summary Much of the offshore of Western and Northwest Europe can be considered as unexplored and underexplored and is thus a major frontier area in terms of oil exploration. If the interior shelf basins of the Celtic Sea and English Channel are excluded, the total number of commercial wells is less than 100 with the majority of these being concentrated in the inner shelf of the Aquitaine, Lusitanian and Faeroe-Shetland Channel Basins. The outer shelf and the upper slope to a realistic depth unit of 1000 m has hardly been tested at all. In these water depths, a significant and relevant contribution to our understanding of the regional petroleum geology has come from the results of DSDP/IPOD drilling on Legs 478, 48, 80 as well as the allied regional seismic surveys made by various European academic and government institutions. In considering the geology of this frontier area, it is appropriate to begin by recognising that the present day distribution and architecture of the principal basins as well as the development of the principal reservoir and source intervals is dependent on the complex history of rifting and spreading that has structured the Atlantic margins north of the Azores-Gibraltar line. Basin development in the area between the Faeroe-Shetland channel and Southern Portugal was initiated in early Permian time along lines influenced by Late Hercynian shears. The first but relatively minor phase of rifting in Triassic-Liassic time reflects the more intense phase that ultimately opened the Atlantic between America and Africa. The next and most major phase of rifting that led to the opening of the north Atlantic was probably initiated in Late Oxfordian time culminating in Late Jurassic-Early Cretaceous time. Spreading had probably begun in the Bay of Biscay, west of Iberia in the Rockall-Trough-Faeroe-Shetland Channel by Aptian time but the kinematics of the opening are not fully understood (Roberts et al. 1981). By Late Cretaceous time, spreading had ceased in the Bay of Biscay and Rockall Trough and a new axis of spreading opened the Labrador Sea. Between Late Cretaceous and Oligocene time, Iberia moved westwards overthrusting the southern Bay of Biscay during the Pyrenean orogeny. In the Lusitanian Basin, Triassic through Liassic sedimentation may have been controlled by the first rifting phase. Late Jurassic and Early Cretaceous sandstones were deposited as a consequence of the main rifting phase. Extension onshore and offshore was controlled in part by the pattern of Late Hercynian shears (Groupe Galice 1979). In contrast to the Lusitanian Basin and the north margin of Biscay, the Iberian margin of Biscay is dominated by overthrust tectonics for the whole of its length. The margin can be simply described as a rifted margin of Late Jurassic-Early Cretaceous age overthrust to the northwest during the Pyrenean orogeny. Principal oil exploration targets in this area include Late Cretaceous turbidites associated with overthrusting and Urgonian limestone deposited the rift phase (Soler et al. 1981). The geological history of the north margin of Biscay and the Gobun Spur has been documented by Roberts et al. (1981). A series of large tilted and rotated fault blocks trending generally NW-SE are thinly covered by post Aptian sediments. These tilted blocks represent the northwestward prolongation of similar features known in the offshore Aquitaine Basin (Montadert et al. 1974). The tilted blocks evidently contain pre and synrift sediments but source and reservoir potential are unknown. Further north the Faeroe-Shetland channel is an area of active exploration. The Channel represents the northward continuation of the Rockall Trough in which there has been no significant exploration activity. Studies of the subsidence history of the Faeroe-Shetland Channel suggest formation by rifting in Late Jurassic-Early Cretaceous time with the onset of thermal subsidence (spreading?) in about Aptian time. During Palaeocene-Eocene time, eruption of lavas from centres in the Faeroes area masked the older section in the channel (Ridd 1981). Established success in the better explored areas of offshore Europe may be a signpost to future success in the underexplored areas. However a full assessment of these areas lies beyond the scope of this review of basin development.

Late Cretaceous-Tertiary stratigraphy, structure, and tectonic history of the area between Whareama and Ngahape, eastern Wairarapa, New Zealand

Abstract A NNE-striking belt of Late Cretaceous-Early Tertiary sediments in the area between Whareama and Ngahape consists of a sequence, from base to top, of: sandstone, conglomerate, and minor siltstone (Glenburn Formation); thin-bedded sandstone and mudstone (Te Mai Formation); siltstone (Whangai Formation); black siltstone (Waipawa Formation); and glauconitic sandstone (Huatokitoki Formation). The belt is faulted against indurated, strongly deformed Early Cretaceous sandstone and siltstone (Pahaoa Group) in the west, and overlain unconformably by Late Tertiary (Miocene) sediments in the east. Shallow intrusions of dolorite and basalt, and submarine basalt flows, comprising the Ngahape Volcanic Complex (new), occur mostly in the upper part of the sequence. Volcanic activity was probably restricted to late Maastrichtian-early Paleocene (Haumurian-Teurian) time. The Late Cretaceous-Early Tertiary formations are considered to have been deposited in inner shelf or marine delta (Glenburn), tidal strait (Te Mai), and shallow restricted basin (Whangai and Waipawa) environments. Major faults, including the new Motuwaireka Fault, in the area mostly trend NNE. Significant discoveries with respect to faults are: repeated movement (late Cretaceous, Pliocene, and Recent?) on Adams Fault, forming the western boundary of the belt; a reverse component of movement on the low-angle (20–40°) west-dipping Pukeroro Fault; and strike-slip (dextral) movement on the ENE-trending Kaumingi Fault. Faulting and folding took place during deposition of Late Cretaceous-Early Tertiary sediments, resulting in penecontemporaneous erosion and slumping. Major tectonic upheaval occurred in post-late Paleocene time, probably in the early Eocene.

A CASE HISTORY OF ROUGH RANGE

Rough Range No. 1 was the first well to be drilled for oil in Western Australia in the post-war period. It was spudded on September 5th, 1953. At a depth of 1100m, the well encountered a flow of oil at a rate of 500 bbls per day from the Early Cretaceous Birdrong Sandstone. This first discovery of flowing oil in Australia set off a boom in exploration for oil that rivalled the gold mining booms of the last century. The discovery well was drilled for a total of 20 months, encountering a section of Tertiary, Cretaceous, Jurassic, Permian, and Carboniferous -? Devonian rocks. By May 1955, when Rough Range No. 1 was terminated, a further seven wells had been drilled on the Rough Range Anticline and all were dry. Late in 1955, Rough Range -9 was drilled but also proved dry. Rough Range-10 was drilled less than 200m from the discovery well but only found a thin, non-commercial pay zone.Studies of the Rough Range structure, incorporating data from all of the wells and the intense seismic mapping of the feature show that the area of closure is very small and the total accumulation could not be more than 282,000 bbls of oil in place in the reservoir.Assuming a 30% recovery factor, the accumulation is capable of producing approximately 84,600 bbls. Of this, 16,900 bbls has already been produced in a 48-day production test of Rough Range-1 A in 1955. WAPET believes that the small remaining reserves, even at world parity pricing, are not economically viable.Although the small accumulation at Rough Range No. 1 was a disappointment to WAPET, this first post-war oil well proved to be a "lucky break" and a big stimulus for oil exploration in Australia.

The Trabuco and Baker Conglomerates of the Santa Ana Mountains

A thick conglomerate series unconformably overlies Triassic metamorphics and Jura-Cretaceous intrusives and probably conformably underlies marine early Upper Cretaceous sandstones. The lower 300-400 feet of this series are deeply weathered, red, soft, massive; the upper 100 feet are fresh, greenish-gray to brown, resistant, thick bedded, and probably marine. The red lower part, named the Trabuco formation by E. L. Packard, was interpreted as a continental deposit older than the overlying gray conglomerate, and by Bernard Moore as a marine deposit continuous with and genetically inseparable from the gray conglomerate but weathered to its present condition by circulating iron-bearing meteoric waters after the deposition of the entire conglomerate series. Field studies of the conglomerates and pebble counts show that the upper and lower conglomerates are probably conformable and have no constant differences in composition, size, or rounding of the included pebbles. The marked and constant differences in co...