--> High Resolution Geochemistry at Well Site, A New Emerging Tool

AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis

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High Resolution Geochemistry at Well Site, A New Emerging Tool


The recent impressive improvements in analytical chemistry, due on one side to the growing needs of environmental monitoring and on the other to fast developments in nanotechnologies, made possible to move part of geochemical lab activities to well site. New portable and more robust instruments, able to replace bulky and complex instruments used in the labs, have been adopted in mud logging units, to introduce geochemistry at well site. This trend is still ongoing and other analytical techniques will be moved soon, offering new opportunities to get in quasi‐real time additional high value data, matching lab quality standards. There are many benefits deriving from moving geochemical analyses to well site: quick responses for optimization of well construction and of real time decisions, fresh samples availability for more accurate analyses, no need of complex procedures for exporting samples and shipping them, less costs or, even better, more data with the same budget. Petroleum system modelling needs the largest possible amount of experimental data to properly constrain results of numerical simulations and well site geochemistry can supply very detailed profiles of different analytical data, crucial for appropriate calibrations. One additional benefit from geochemistry at well site is flexibility in sampling, based on analyses results. Cutting samples are often taken at regular intervals, defined in advance by well program: sampling frequency is sometimes similar in very homogeneous and less important sequences as well as in highly heterogeneous formations such as reservoir and source rocks. By using geochemistry at well site and evaluating results in real time, it is possible to change and to adapt sampling intervals, taking more samples or different type of samples, where heterogeneities are evident or, more in general, in the key formational intervals. Thanks to lower cost, and to the consequent possibility to get more samples and analyses, and to the flexibility in sampling, a high definition integrated picture of reservoirs and of source rocks to be used as input to PSM exercises can be obtained. “Integrated” stands for a stringent integration of fluid compositions and rock properties, to investigate and to correlate fluid and rock heterogeneities. Among the different discussed examples, one is originated by the integration of XRD (X‐ray diffraction) with XRF (X ray fluorescence) data, to get a high‐resolution chemostratigraphical picture of different formations. To better constrain basin geometrical evolution through time, inter‐wells correlations could be necessary for more detailed reconstructions of sedimentation processes and chemostratigraphy based on XRF and XRD can be fruitfully used at this aim. Ratios between different chemical elements, due to specific paleo‐environmental conditions limited in time, can differentiate homogeneous and uniform sedimentary sequences, allowing inter‐well correlations using intra‐formation horizons. Other ratios can be used for reservoir zonation distinguishing different facies or different level of diagenesis. To detect elemental ratios anomalies, it is very often necessary to have a high frequency sampling strategy, that only wellsite approach makes possible. Another example is coming from integration of Thermal Extraction – Gas Chromatography (TE‐GC) with XRD and XRF data to associate different type of oils with different facies in the reservoir or to evaluate the extent of biodegradation and what part of the reservoir is affected by this phenomenon. An interesting case history shows that a GC analysis of a produced oil has a bimodal distribution, interpreted as a mix of two maturity fluids. By means of well site geochemistry it was possible to assess what levels were producing different types of oils and their lithological characteristics and to identify a sub interval with higher density oil. Geochemical analyses are always integrated with mud logging data, that can be considered as part of geochemistry, adding important information about gas and light hydrocarbons composition.