3 research outputs found
Geological risk and reservoir quality in hydrocarbon exploration
In the next 20 years, the global demand for oil is forecast to grow by 0.7% every year,
and the demand for natural gas will increase by 1.6% annually. But as we continue to
produce oil and gas, the resources of our current oilfields are depleting. To meet the
rising global energy demand, it is essential that we can keep discovering more
petroleum resources in the future.
The primary aim of this PhD project is to deepen our understanding of hydrocarbon
reservoirs and enhance our ability to explore. The first project looked at the geological
risks in hydrocarbon exploration. It reviewed and statistically analysed the data of 382
unsuccessful boreholes in the UK offshore area. The results suggest that the most
significant risk for an exploration well is encountering a thin or absent target reservoir.
This risk happened to 27 ± 4% of the past unsuccessful wells. The following most
common risks are low-porosity reservoirs (22 ± 4% of all cases) and the lack of a
closed trap (23 ± 4%). The probability of a target reservoir having a leaky caprock is
5 ± 2%. The study has calculated the probability of occurrence of all the geological
risks in exploration, and this risk data can be applied to predict the potential geological
risks in future exploration.
One challenge in developing saline aquifers as CO2 storage reservoirs is the lack of
subsurface data, unless a well has been drilled. Drawing on the experience of
hydrocarbon exploration, a potential CO2 storage site identified on seismic profiles will
be subject to many uncertainties, such as thin or low-porosity reservoirs, leaky seals,
which are analogue to the geological risks of an undrilled hydrocarbon prospect. Since
the workflow of locating CO2 storage reservoirs is similar to the exploration for
hydrocarbon reservoirs, the risk data of hydrocarbon exploration wells can be applied
to infer the geological risks of the exploration wells for CO2 storage reservoirs. Based
on this assumption, the study of Chapter 3 estimated that the probability of a borehole
encountering a reservoir suitable for CO2 storage is c. 41–57% (90% confidence
interval). For reservoirs with stratigraphic traps within the UKCS, the probability of
success is slightly lower, at 39 ± 10% (90% confidence).
Chapter 4 studies the porosity and diagenetic process of the Middle Jurassic Pentland
Formation in the North Sea. The analysis data come from 21 wells that drilled and
cored the Pentland Formation. Petrographic data suggest the content of detrital illite
is the most important factor affecting the porosity of the Pentland Sandstone - the
porosities of the sandstones with more than 15% of illite (determined by point-count)
are invariably low (< 10%). Quartz cement grows at an average rate of 2.3 %/km
below the depth of 2km, and it is the main porosity occluding phase in the deep
Pentland Sandstone. Petrographic data shows the clean, fine-grained sandstones
contain the highest amount of quartz cement. Only 1-2 % of K-feldspar seems to have
dissolved in the deep Pentland Sandstone (> 2 km), and petrographic data suggest
that K-feldspar dissolution does not have any substantial influence on the sandstone
porosity. There is no geochemical evidence for mass transfer between the
sandstones and shales of the Pentland Formation.
Chapter 5 investigates the high porosity of the Pentland Sandstone in the Kessog
Field, Central North Sea. The upper part of the Kessog reservoir displays an
anomalously high porosity (c. 25 %, helium porosity) that is 10 % higher than the
porosity of other Pentland sandstones at the same depth (c. 15 %, 4.1 - 4.4 km).
Petrographic data show these high porosities are predominantly primary porosity. The
effects of sedimentary facies, grain coats, secondary porosity and overpressure on
the formation of the high porosity are considered to be negligible in this case. Early
hydrocarbon emplacement is the only explanation for the high porosity. In addition to
less quartz cement, the high-porosity sandstones also contain more K-feldspar and
less kaolin than the medium-porosity sandstones of the same field. This indicates that
early hydrocarbon emplacement has also inhibited the replacement of K-feldspar.
The last chapter studies the potential mass transfer of silica, aluminium, potassium,
iron, magenesium and calcium at sandstone-shale contacts. The study samples
include 18 groups of sandstones and shales that were collected from five oilfields in
the North Sea. The interval space between the samples of each group varies from
centimetres to meters. The research aim is to find evidence of mass transfer by
studying the samples’ variation of mineralogy and chemistry as a function of the
distance to the nearest sandstone-shale contact. The sandstones are mostly turbidite
sandstones, and the shales are Kimmeridge Clay shales. Petrographic, mineralogical
and chemical data do not provide firm evidence for mass transfer within any group of
the samples. The result indicates that the scale of mobility of silica, aluminium,
potassium, iron, magenesium and calcium in the subsurface may be below the scale
of detection of the study method, i.e. < 5 cm
Emergent quality issues in the supply of Chinese medicinal plants: A mixed methods investigation of their contemporary occurrence and historical persistence
Quality issues that emerged centuries ago in Chinese medicinal plants (CMP) were investigated to explore why they still persist in an era of advanced analytical testing and extensive legislation so that a solution to improve CMP quality could be proposed.
This is important for 85% of the world’s population who rely on medicinal plants (MP) for primary healthcare considering the adverse events, including fatalities that arise from such quality issues. CMP are the most prevalent medicinal plants globally.
This investigation used mixed-methods, including 15 interviews with CMP expert key informants (KI), together with thematic analysis that identified the main CMP quality issues, why they persisted, and informed solutions. An unexplained case example, Eleutherococcus nodiflorus (EN), was analysed by collection of 106 samples of EN, its known toxic adulterant Periploca sepium (PS), and a related substitute, Eleutherococcus senticosus (ES), across mainland China, Taiwan and the UK. Authenticity of the samples was determined using High-performance thinlayer chromatography.
Misidentification, adulteration, substitution and toxicity were the main CMP quality issues identified. Adulteration was found widespread globally with 57.4% EN found authentic, and 24.6% adulterated with cardiotoxic PS, mostly at markets and traditional pharmacies. The EN study further highlighted that the reason CMP quality issues persisted was due to the laboratory-bound nature of analytical methods and testing currently used that leave gaps in detection throughout much of the supply chain.
CMP quality could be more effectively tested with patented analytical technology (PAT) and simpler field-based testing including indicator strip tests. Education highlighting the long-term economic value and communal benefit of delivering better quality CMP to consumers was recommended in favour of the financial motivation for actions that lead to the persistence of well-known and recurrent CMP quality issues