Text 15

contemporaneously – одночасно

unconformity – неузгоджене залягання

truncation – усічення, врізання

subcrop – попереднє оголення

rift – тріщина, розколина

subsidence – падіння

tensional – напружений

Palaeogeomorphic traps are traps that resulted from the accumulation of sediment over a pre-existing topography, the physiographic expression of which led to facies that could generate reservoir rock and, usually contemporaneously, facies that would eventually act as cap rock. These are almost in­variably diachronous, transgressive sequences that include petroleum source rock.

Unconformity traps are those that resulted from the truncation of reservoir rocks and the subsequent sealing of the subcrop by an unconformable, relatively impermeable, fine-grained, rock unit. The source rocks may be within the pre-unconformity sequence, or in the immediate post-unconformity cap rocks. The timing of secondary migration is not, of course, earlier than the time of sealing of the subcrop.

Of particular importance to geology is the occurrence of both types of trap in a world-wide geological context related to the Mesozoic development of rift margins to continents (e.g. North-West Europe, Alaska, Australia) and the development of rift basins that are not parallel to a continental margin (e.g. North Africa). These areas show very similar geological histories, begin­ning with rifting and growth faulting, followed by transgression and subsidence with little or no fault movement. Commonly the development began in the Permian or Triassic and continued well into the Tertiary, at least. Not all such areas have strictly contemporaneous events, but most show a prolonged "tensional" regime that began long before the opening of the adjacent ocean, and continued while the ocean opened wider.

Text 16

diaper – діапіровіт

intrusion – інтрузія (магми)

incipient – початковий

hence – отже

buoyancy – плавучість

elongate – видовжений

deficiency – відсутність

overburden – покривна порода; нанос

inhibit – перешкоджати; стримувати, обтяжувати

hiatus – пропуск

expulsion - виключення

Diapirs,in the context of petroleum geology, are intrusions of sedimentary rocks, primarily salt or mudstone, into the overlying sedimentary sequence. Insipient diapirs are salt pillows and the analogous mudstone pillows or “shale masses”. Deformation of the sedimentary rocks around and above diapirs and incipient diapirs creates potential petroleum traps.

Diapirs are initiated by unequal loading of a layer of material of small equivalent viscosity. The common diapiric materials – salt and abnormally pressured mudstone – may be less dense than the normally compacted sedimentary rocks overlying them. Hence, once a diapir has been initiated (particularly a salt diapir), the forces of buoyancy tend to elongate the deformation vertically.

The upward movement of a diapir is relative to the surrounding sedimentary rocks. The accumulation of a sedimentary sequence over a diapir indicates that it was subsiding with the development of the sedimentary basin. The upward movement is only absolute if the relative movement is faster than subsidence of the surrounding sedimentary sequence.

This differential subsidence may influence the accumulation of sediments, contributing to the variations of loading on the diapiric mother bed.

The mechanical properties of the diapiric material change with time and position. Salt becomes less viscous with increasing temperature. Mudstone viscosity is a function of pore pressure and depth as well as temperature.

A diapir commonly, but not invariably, shows a gravity minimum. This indicates a deficiency of mass.

Failure of the overburden by faulting may accompany diapiric development; but diapiric development may also inhibit subsidence locally at the surface of accumulation sediment and so lead to a local stratigraphic hiatus.

Diapirism is necessarily contemporaneous with the expulsion of pore fluids from compacting mudstones. Diapiric mudstone may also be a petroleum source rock.