Peter E. Putnam

Estuarine deposits of the upper Qishn Formation (Lower Cretaceous), Masila region, Yemen

In the Masila region of Yemen, several hundred million barrels of recoverable oil occur within three Lower Cretaceous siliciclastic lithostratigraphic units of the upper Qishn Formation. The basal unit, S3, locally exceeds over 40 m in thickness. This unit overlies an erosive surface and is formed of blanket deposits of cross-bedded, fining-upward, pebbly sandstones with siliceous and dolomitic paleosols. The S3 unit reflects deposition from coarse-grained fluvial channels and related environments. The middle unit, S2, is composed of complexly interbedded sandstones and mudstones up to over 25 m thick. These deposits grade upward from S3 sediments and display cross-bedded pebbly sandstones, cross-bedded sandstones with mud drapes, and inclined heterolithic units formed of interbedded sandstones and mudstones. The S2 unit is capped by widespread churned and rooted mudstones 3-5 m thick. S2 deposition resulted from the rapid infilling of an estuary, or estuaries, created by the transgression of the fluvial systems responsible for S3 sedimentation. The uppermost unit, S1, at its base is formed of an eastern shelfal package with Helminthopsis- and Chondrites-bearing mudrocks, and western beds formed of sediments generally similar in appearance to the middle S2 unit. Above the lowest S1 deposits are marine muddy coquinas that blanket the study area. In the west, these sediments are abruptly to gradationally overlain by Ophiomorpha-bearing, fine-grained, cross-bedded shelly sandstones up to about 10 m thick. These sandstones form a north-south linear trend and are interpreted to represent a progradational pulse of subtidal shoal sediments that interrupted the general deepening trend exhibited by the upper Qishn interval. Final upper Qishn deposition was younger shelfal muddy coquinas that overlie the shoal deposits. These and younger carbonate sediments act as the uppermost seal to reservoirs in the Masila region. The upper Qishn sediments are compared to modern and Pleistocene estuarine deposits. We propose stratigraphic, depositional, and paleogeographic models for this formation and, from this analysis, we derive several implications for future petroleum operations in the region.

Fluvial Channel Sandstones Within Upper Mannville (Albian) of Lloydminster Area, Canada--Geometry, Petrography, and Paleogeographic Implications

Within the Albian upper Mannville of the central Canadian plains there exists a complex of anastomosing channel sandstones which are oriented northwest-southeast and which can be traced for over 300 km. The paleoflow direction was to the north. The channel sandstones exhibit a complex mineralogy with separate quartzose and lithofeldspathic channel fills present. When viewed within the regional geologic framework of the Lower Cretaceous of western North America, the channel orientation combined with the sandstone petrography indicates that the main sediment source was the Rocky Mountains of the northwest United States. Such a conclusion is in contrast to previous studies of the upper Mannville of the Canadian plains which concluded that the Canadian Cordillera was the major sediment source for an eastwardly prograding clastic wedge.

Anastomosed River Deposits--Modern and Ancient Examples in Western Canada: ABSTRACT

Depositional facies of two Canadian modern anastomosed river systems, the upper Columbia River and lower Saskatchewan River, occur in intermontane and plains settings, respectively. Both systems contain low-gradient, multiple, interconnected, laterally stable sand-bed channels, with adjacent splay, levee, and shallow wetland deposits, all aggrading in accordance with channel sedimentation. While aggrading cross-valley alluvial fans or subsidence tend to control sedimentation rates in intermontane valleys, basin subsidence and/or regional tilting controls deposition rates in plains settings. Deposits in the upper Columbia River valley (120 × 1.5 km) consist of low-sinuosity multistoried stringers (textural cycles) with planar tabular cross-bed sets of channel sands and numerous sandy crevasse-splay deposits. Channel deposits are laterally contained by deposits of levee silt and lacustrine mud, and when buried are vertically mud encased. Aggrading at an average rate of 60 cm/100 years over the past 2,500 years, the anastomosed system is very dynamic, exhibiting many avulsions and channel End_Page 631------------------------------ fills. Deposits in the lower Saskatchewan River valley (120 × 80 km), a much wider basin with much slower aggradation rates (± 15 cm/100 years), occur as channel sands flanked by laterally extensive (1 km) sheets of overbank levee deposits of fine sandy silt, which grade into even more laterally extensive thick deposits of mud or peat. With time, dominant channels become sinuous, thus causing increased flow resistance, major avulsions upriver, and eventual channel filling and abandonment. Many channel sand-fill deposits form cross-section geometries, ranging up to 15 cm thick by 120 m wide. Facies differences of the two anastomosed river systems are believed to be caused by both the rate of sedimentation and width of the sedimentary basin. Other than size differences of similar sedimentary environments, Columbia River channels are less sinuous, avulse more frequently, and contain coarser grained sand. In the Saskatchewan, some crevasse-splay (sheet sands) and associated avulsions are laterally extensive (10 × 30 km) and very complex. Wetlands in the Columbia are dominated by marsh (organic-rich mud) and lacustrine silt, whereas thick (up to 3 m) laterally extensive peat bogs dominate in the Saskatchewan system. Within the upper Mannville Group of the Lloydminster area there exists a large-scale mappable complex of fluvial channel-fill sandstones that exhibit an anastomosed pattern. The complex has areal dimensions of 250 km (width) by 700 km (length). Channel sandstones are thick (up to 35 km), narrow (300 m), can be traced for several kilometers, and are stratigraphically variable. The channel fills are multistoried, with the predominant sedimentary structures consisting of plane beds, cross-beds, and climbing current ripples. Interchannel sediments consist of interbedded sheet sandstones, siltstones, mudstones, and coals. The predominant sedimentary structures of the interchannel sandstones are the same as those found within channel sandstones. From a compare-and-contrast approach, it is concluded that meandering, sandy braided, valley-fill, deltaic, or tidal origins cannot account for the observed sand-body geometries and facies distribution. The modern model that best explains the sediment and facies distributions within the upper Mannville is the anastomosed fluvial model in which narrow, vertically accreting channels are bordered by extensive aggrading interchannel wetland deposits with interbedded crevasse-splay sands. Hydrocarbon distributions within the upper Mannville are stratigraphically controlled and oil quality can be directly related to depositional facies. Common trapping mechanisms consist of updip shale-filled channels, structural closure formed by differential compaction, and lateral sandstone pinchouts. End_of_Article - Last_Page 632------------

Mannville Group of Lloydminster Heavy Oil Fields, Canada--Depositional Overview: ABSTRACT

A regional subsurface study of 200 cores and several thousand well logs has led to an interpretation of the depositional history for the Lloydminster area of western Canada. Major geologic controls on deposition were tectonism in western North America, a series of northwest-trending ridges formed of Paleozoic carbonates found in the central plains, salt collapse within the depositional basin, and eustatic changes associated with Boreal and Gulfian seas. On the basis of genetic units, the Mannville Group has been divided into three units: lower, middle, and upper. The lower unit is dominantly a siliceous, fine to medium-grained sandstone which is found in the lows between the northwest-trending ridges of Paleozoic carbonates. The unit is up to 60 m thick with the dominant sedimentary structures being high-angle (30°) cross-stratification. The middle unit consists mainly of upward-coarsening, very fine to fine-grained, quartzose sheet sandstones, 6 to 10 m thick, associated with a restricted marine microflora and microfauna. A typical sequence begins with a basal bioturbated shaly siltstone and proceeds upward through wavy lenticular sandstones in silty shales, wave-rippled sandstones with mud drapes, wavy-bedded sandstones, and low-angle (0 to 10°) cross-laminated sandstones. This sequence is commonly capped by a coal or gray carbonaceous shale. Although sheetlike in appearance, the lateral continuity of the sandstones is commonly broken by shale or sand-filled channels. The upper unit is comprised of thick (up to 35 m) lenticular channel fills of cross-bedded, lithofeldspathic or quartzose sandstones which grade laterally into interbedded deposits of current-rippled sheet sandstones, siltstones, mudstones, and coals. The vertical sequence of environments is interpreted to be: post-Paleozoic incision followed by valley-fill deposition of an aggrading north-flowing fluvial system (lower unit), transgression followed by regressive sedimentation in wave-dominated paralic environments (middle unit), and continued regressive deposition within an areally extensive, north-flowing anastomosed fluvial network (upper unit). Mannville deposition in the Lloydminster area was terminated by a major transgression which deposited the widespread marine shales of the Colorado Group. End_of_Article - Last_Page 620------------