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日本堆積学会　メールマガジン　◆2024年9月27日（1202号）
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日本堆積学会の皆様

10/22（火）に、石油技術協会砂岩分科会と日本堆積学会との共催の講演会を、
下記の通り、開催します。本講演会は，日本堆積学会産官学連携推進委員会の
活動の一環（産学連携セミナー）として、砂岩分科会との共催で行うものです。
参加ご希望の方は、下記の申し込みURLから10/18（金）までにお申込み下さい。

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石油技術協会 砂岩分科会・日本堆積学会　共催　講演会

【開催案内】
石油技術協会砂岩分科会　令和6年度第1回講演会

日　時：10月22日（火）日本 15:00- / UK 7:00-（2時間程度）
講演者：Andrew Hurst教授（英国アバディーン大学）
内　容：Sand Injectites in Sedimentary Basins: Their Significance in 
        Basin Evolution & Hydrocarbon Prospectivity
形　式：オンライン（Microsoft Teams）
共　催：日本堆積学会

登　録：Microsoft Forms（10月18日（金）締切）
https://forms.office.com/r/8uSQdzyvMd

要　旨：
SAND INJECTITES IN SEDIMENTARY BASINS: THEIR SIGNIFICANCE IN BASIN 
EVOLUTION & HYDROCARBON PROSPECTIVITY

Andrew Hurst, Professor of Energy Geoscience, University of Aberdeen

First described in 1827, the forceful injection of sand into finer 
grained strata, failed to attract more than casual interest in the 
geological community until the fortuitous recognition of sandstone 
intrusions as hydrocarbon reservoirs on the UKCS in the early 1990’s. 
Several similar fortuitous discoveries were made adjacent to Harding 
field, but the first deliberate exploration of a sand injection complex 
did not take place until 2003-4, when Volund field was discovered (NCS). 
In the early 1990’s to early 2000’s, the significance of sandstone 
intrusions in several oil fields in development became apparent and 
changed field development strategy with respect to incremental reserves 
and their distribution and enhancing recovery by optimising well-placement. 
In the North Sea more than 11 bn bbls oil are known to occur in, or are 
associated with, sand injection complexes; in the North Sea 2 bn bbls oil 
occur in reservoirs where sandstone intrusions predominate.
Sandstone intrusions have varying levels of discordance with bedding, which 
contributes significantly to the ease of differentiating sandstone intrusions 
from depositional sandstones on seismic data. Individual sandstone intrusions 
that form significant reservoir volumes (10’s m thick) often occur above, 
within and below biozones. Characteristics of the external geometry of 
sandstone intrusions includes steps, bifurcation, upward erosion of intrusion 
margins, preservation of fracture margins, and partial disintegration of host 
strata. These combine to form composite sedimentary intrusions, forming 
saucer-shaped intrusions, wings, sill complexes and dyke swarms, which form 
regionally in the subsurface and outcrop, over scales of 1000’s km2 and often 
>1km thick.
Examples of large sandstone intrusions from the subsurface will be shown together 
with similar scale outcrop examples. Comparison of these data will be used to 
illustrate the limitations of seismic imaging when defining the gross volume, N/G 
(storage capacity and reservoir quality), presence of sub-seismic reservoir, 
lateral and vertical connectivity and fluid contacts. Core-scale characteristics 
are sometimes diagnostic of sandstone intrusions and occur in core samples and 
outcrop, several of which record the hydrofracture of host mudstone. Angular 
fractures on grain surfaces and intra-crystalline micro-fracture networks are 
diagnostic of sandstone intrusions. Grain textures record multiple high-velocity 
inter-granular collisions during sand injection in which turbulent flow prevails 
in dilute granular suspensions.
Pore-fluid pressure (Pf) is requisite to the formation of sand injection complexes 
such that Pf exceeds the fracture gradient and ultimately the lithostatic gradient 
+ the tensile strength of the seal. Absolute Pf is less critical than the contrast 
between the hydrostatic and lithostatic gradients, a contrast that is least during 
shallow burial sandstone intrusions (assuming a linear hydrostatic gradient). When 
the seal fails, hydrofractures form and propagate toward a free surface. Initially 
the hydrofractures are fluid filled but as they propagate fluid velocity increases, 
which when it exceeds the minimum fluidisation velocity for sand begins to entrain 
fine to medium grained grains, which form sandstone intrusions in some of the 
hydrofractures. There are several mechanisms by which Pf can create overpressure, 
but formation of giant injection complexes requires large volumes of fluid, likely 
to be derived by regional burial processes and probably most likely associated with 
chemical diagenesis.

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