 Data Analysis - Case Study 5
Lithuania
“Reservoir sweet spots identified with surface geochemistry
– an example from the
Cambrian Pietu Siupariai oil field, Lithuania.”
Authors: T. Haselton and P. Willumsen
Summary
A surface geochemical survey was conducted over the ca. 2,000 m deep Cambrian Pietu Siupariai oil field, Lithuania, with the objective to identify reservoir sweet spots within the
closing contour. For that purpose a total of 99 absorbing field modules (GORE-SORBER® modules) were placed in a 250 by 500 m grid within a 20 Km2 area over the field and exposed for 17 days at a depth of 1 m below grade. Also 30 modules were installed at three wells –one
producer, one dry well and one with shows- in order to establish a geochemical model.
Hydrocarbons absorbed within the modules –typically in nannogram range quantities - were
analysed for a spectrum of 72 different hydrocarbon compounds by GCMS. Probability maps displaying the similarity to the geochemical model were generated by statistically relating the
field sample fingerprints and the geochemical model.
The results show a very good correlation between the geochemical probability maps and test/production data from wells within the field (R2= 0.96). They also show that production
varies considerably between wells within the closing contour, namely in the range from 0 to 160
BOPD in past wells, indicating inhomogeneous reservoir characteristics.
After the geochemical survey was completed three wells were drilled on positive geochemical
anomalies within the field. These wells produced up to 20 times better than the previously best
producer in the field.
Introduction
The Pietu Siupariai field is located in the Baltic Syneclise petroleum province close to the Baltic
coast in Lithuania. The petroleum system consists of Cambrian source rock and reservoir with
traps formed by Caledonian tectonism (ref.1). Petroleum reservoirs consist of Cambrian
sandstones deposited in a near-shore marine environment. The oils in the region have a uniform
composition with API gravities in the 40 to 43 range.
Several fields have been discovered in the region, which has been in production since 1990. The fields are located adjacent to a regional sealing fault. The 3 by 6 Km Pietu Siupariai field had
until the end of 1999 produced a total of 0.2 MMBO from 3 wells. The Cambrian Daimena
Formation reservoir is 70 meters thick in the field and has porosity in the 5 to 15 % range and
permeability from 0.1 to 400 milliDarcy. The relatively sparse well control indicates NE-SW
oriented reservoir quality trends parallel to the paleo-coastline. The structure has a relief of only
30 meters above the 1970-meter sub sea oil/water contact. Productivity of the old wells in the
field varies considerably from no production in the G-14 well to 160 BOPD in the PS-1 well.
The G-12 well tested 750 BOPD on a DST but was not completed due to mechanical problems.
Although the G-18 and G-7 wells both produced 120 BOPD, the latter well only penetrated the
uppermost 15 meters of the reservoir.
Methods
In 1999 a drilling campaign was planned to increase field production, and a surface geochemical
survey was conducted to guide the selection of drilling targets. The selected technology was
“GORE-SORBER® Exploration Survey”, a method developed by W.L. Gore & Associates, Inc.
This technology is a passive soil gas method. Samples are collected by exposing absorbing
modules at a depth of 1 meter below grade for 17 days. The modules are laid out in a regular grid
compatible with the objective of the survey. In the case of the Pietu Siupariai survey the grid size
was 500 by 250 meters within a 20 Km2 area, and a total of 99 grid modules were installed over
the field. Additionally modules were installed around existing wells to define a geochemical
model against which to evaluate the grid modules. In this survey the dry G-13 well was used as a
negative model point expressing the background signature, the G-6 oil well as a positive model
point and the G-14 well with oil shows was used as an intermediate model point. Ten absorbing
modules were installed at each of these wells.
After retrieval the modules were analysed by GCMS for a range of 72 hydrocarbon compounds in the C2 to C18 range. The amount of absorbed hydrocarbons typically is in the nannogram range
(10–9 gram) per module. However, it is not the amount of hydrocarbons, but the relative
distribution of compounds present (the “fingerprint”) that is used in the evaluation of the results.
First the geochemical end points are defined by comparing the signatures among the modules
around the dry well and the oil well, respectively. The dry and oil well signatures are
subsequently compared in order to generate the end points of a geochemical model for the area.
Secondly each grid modules is compared to the geochemical model, resulting in a probability
value expressing the degree of similarity between the two. Simplistically put one ends up with
the petroliferous signature by subtracting the dry background signature from the grid module
signature. The results are presented as colour contoured probability maps over the survey area,
highlighting positive and negative geochemical anomalies.
The GORE-SORBER® Exploration Survey method has been proven to give good results in a
wide range of geological and environmental conditions, and the results have been demonstrated
to reduce exploration risk considerably. Based on the results of 143 wells Potter et al (ref. 2)
have documented that wells drilled on positive geochemical anomalies actually encountered
hydrocarbons in more than 9 times out of 10. Conversely wells drilled on negative geochemical
anomalies were dry 9 times out of 10. This contrasts sharply with the average world exploration
success of about 1 in 10.
The theoretical background for the GORE-SORBER® Exploration Survey method –as for surface geochemical methods in general- is that all hydrocarbon accumulations leak minute
quantities of hydrocarbons to the surface, either by way of micro bubbles (ref. 3) or as
continuous phase flow through fractures (ref. 4). The geochemical composition recorded at
surface is usually leaner than the underlying hydrocarbon accumulation, presumably due to
fractionation and other secondary alteration mechanisms during the migration process. In
practice no seal is so tight as to be able to prevent micro seepage. This is demonstrated most
clearly by an example from Egypt, where oil bearing and dry fault blocks were imaged by a
surface geochemical survey through 8,000 feet of salt and anhydrite (ref. 5).
Results
The geochemical probability map of the Pietu Siupariai field is shown in Figure 1. The map
shows the low relief structure bounded by a fault to the east, and having the oil/water contact at a
sub sea depth of 1970 meters. The map also shows a girdle of high probability values (positive
geochemical anomalies) within the closing contour –for example around the G-12 and G-6/PS-1
wells- and actually extending beyond it at some points. Outside the closing contour, and in
certain areas within it (for example around the G-14 well), the maps show low probability values
(negative geochemical anomalies).
Based on the probability map it would be expected that wells drilled on negative anomalies
would be dry or have low productivity –even within the closing contour-, and that wells drilled
on positive anomalies would be hydrocarbon bearing and productive. A comparison of well
results and the probability map shows this to be true (see Figure 1): the G-14 well within the
closing contour and with a probability value of 41 had oil shows but no production; the G-18
well has a production of 120 BOPD and a probability of around 62; the G-12 and G-7 wells both
have probability values in the 75 range; G-12 tested 120 BOPD but was not completed for
mechanical reasons; G-7 produces 120 BOPD from the upper 15 meters of the reservoir. The G-
6/PS-1 well has a probability value of 76 and produced 160 BOPD at the time of the survey.
The PS-2, PS-3 and PS-4 wells were drilled subsequent to the surface geochemical survey on
positive geochemical anomalies with probability values of 100, 97 and 90, respectively. The
initial production from the Cambrian Daimena reservoir in these three wells was 3350, 2020 and
760 BOPD, respectively, making them the best producing wells in the field and indeed in the
Baltic region. The results from the new wells are consistent with the relationship between
positive geochemical anomalies on the one side and good reservoir performance in the field on
the other. It is expected that similar results could be obtained over other fields with
inhomogeneous reservoir characteristics, optimising the development program by reducing the
number of poor producing wells.
Conclusions
The survey conducted over the Pietu Siupariai field in Lithuania demonstrates that it is possible
to use surface geochemistry to determine reservoir sweet spots within a producing oil field.
Based on a comparison of well and surface geochemical data it has, in fact, been possible to
establish a good exponential fit with an R2 0.96 between surface geochemical probability values
and well productivity (Figure 1). In consequence it appears possible to quite accurately predict
subsurface reservoir quality variations with surface geochemistry.
References:
1) Stirpeika, A. (1999): “ Tectonic Evolution of the Baltic Syneclise and Local Structures in the South Baltic Region
with Respect to their Petroleum Potential”. Geological Survey of Lithuania, Vilnius.
2) Potter et al (1996): “Significance of Geochemical Anomalies in Hydrocarbon Exploration: One Company’s
Experience”, in Schumacher & Abrams, eds., “Hydrocarbon migration and its near-surface expression”: AAPG
Memoir 66, p. 431-439.
3) Klusman et al (1996): “Comparison of Light Hydrocarbon Microseepage Mechanisms”. in Schumacher &
Abrams, eds., “Hydrocarbon migration and its near-surface expression”: AAPG Memoir 66, p. 157-168.
4) Brown, A. (2000): “Evaluation of Possible Gas Microseepage Mechanisms”. AAPG Bulletin, v. 84, No 11.
5) El Bishlawy et al (2001): “Geochemistry and Surface Detection of Hydrocarbon Microseepage Through Thick
Evaporitic Sequences, Gulf of Suez, Egypt” (Oil & Gas Journal, May 7, 2001)
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