In El Huemul field, four main subvertical features have been described where the continuity of seismic reflectors and reflection amplitude are relatively low causing a diffuse character of seismic data. These chimney-like features are mostly linked to mafic igneous rocks, corresponding to both dikes and sills. Geobodies extracted by means of simple seismic attributes allow a three-dimensional view of these features and igneous bodies for morphological analysis.
The sizes of the intrusive bodies are disproportionately low compared with the one of diffuse features. In fact, many wells have been drilled through them without contacting igneous rocks, and with some evidence of gas migration: 1-Mudlogging service describes oil-filling microfractures in tuffaceous shales, which represent the non-productive lithology in Mina del Carmen Fm (MDC). 2-This produces an increase of heavy alkane gases proportion and higher total mud gas readings. 3-The biggest two features are related to "shallow gas" production. 4-Overpressure in MDC Fm. has been documented only in one of these zones. 5-Hydrocarbon gases sampled from the source rock (Pozo D-129 Fm.) in a well located in one of the seismically diffuse features, are a mix of primary and secondary postmature (VRE: 1.8%) thermogenic gases according to the chemical composition and stable isotopes analyses. This maturity is not consistent with the one corresponding to the producing organic-rich black shales (Ro: 0.93–1.22%). In addition, CH¾He ratios suggest that 10% abiogenic mantle methane might be mixed. 6- Maximum CO2 values were registered, not only by means of mudlogging but also by well tests. 7- The δ13CO2 values and CO⅔He ratios obtained from the same samples referred, allow us to state that this gas is unequivocally within the range of magmatic CO2.
These characteristics, added to the diffuse seismic image, lead to suggest that these features represent fluid migration pathways. Nevertheless, worldwide chimneys are originated in an organic matter-rich source rock, features described here begin in the basement. We interpret that they were triggered by Cenozoic magmatism that directly yielded to CO2 migration and contribute to the local increase of the source rock maturity, enabling hydrocarbon migration.
The characterization of these features allows the identification of CO2 risk zones, which is a key factor when planning the exploration and development of both conventional and unconventional reservoirs.
