A tool assembly comprising: a tool body defining a tool axis, the tool body configured to be receivable in a borehole defined by a borehole wall; a stabilizer mounted on the tool body, the stabilizer displaceable in a direction transverse to the tool axis; an actuating mechanism coupled to the stabilizer and configured to move the stabilizer between: a retracted position in which the stabilizer is spaced from the borehole wall, and a deployed position in which a contact surface of the stabilizer engages the borehole wall at a first location; a seal carried by a seal piston mounted on the stabilizer and configured to sealingly engage the borehole wall at a second location when the stabilizer is in the deployed position to define a sealed isolation zone isolated from borehole fluids; wherein the second location is spaced from the first location; a testing mechanism within the tool body and configured for testing one or more formation properties in the isolation zone; a seal actuating mechanism including a hydraulic system to move the seal piston configured to hydraulically actuate displacement of the seal on the seal piston radially against the borehole wall when the stabilizer is in the deployed position, the seal actuating mechanism acting between the stabilizer and the seal piston and wherein the seal actuating mechanism is configured to continuously urge the seal into sealing contact with the borehole wall; wherein the seal piston is annular and is received in a complementary annular piston chamber in a radial end face of the stabilizer, and characterized in that the piston chamber contains hydraulic fluid and is in fluid communication with the stabilizer via fluid flow lines to form the hydraulic system configured to control the pressurization and expansion of the piston chamber and the piston chamber further comprises a circumferentially spaced series of elastic tension springs connected between the stabilizer and the seal piston. a tool body defining a tool axis, the tool body configured to be receivable in a borehole defined by a borehole wall; a stabilizer mounted on the tool body, the stabilizer displaceable in a direction transverse to the tool axis; an actuating mechanism coupled to the stabilizer and configured to move the stabilizer between: a retracted position in which the stabilizer is spaced from the borehole wall, and a deployed position in which a contact surface of the stabilizer engages the borehole wall at a first location; a retracted position in which the stabilizer is spaced from the borehole wall, and a deployed position in which a contact surface of the stabilizer engages the borehole wall at a first location; a seal carried by a seal piston mounted on the stabilizer and configured to sealingly engage the borehole wall at a second location when the stabilizer is in the deployed position to define a sealed isolation zone isolated from borehole fluids; wherein the second location is spaced from the first location; a testing mechanism within the tool body and configured for testing one or more formation properties in the isolation zone; a seal actuating mechanism including a hydraulic system to move the seal piston configured to hydraulically actuate displacement of the seal on the seal piston radially against the borehole wall when the stabilizer is in the deployed position, the seal actuating mechanism acting between the stabilizer and the seal piston and wherein the seal actuating mechanism is configured to continuously urge the seal into sealing contact with the borehole wall; wherein the seal piston is annular and is received in a complementary annular piston chamber in a radial end face of the stabilizer, and characterized in that the piston chamber contains hydraulic fluid and is in fluid communication with the stabilizer via fluid flow lines to form the hydraulic system configured to control the pressurization and expansion of the piston chamber and the piston chamber further comprises a circumferentially spaced series of elastic tension springs connected between the stabilizer and the seal piston. piston chamber in a radial end face of the stabilizer, and characterized in that the piston chamber contains hydraulic fluid and is in fluid communication with the stabilizer via fluid flow lines to form the hydraulic system configured to control the pressurization and expansion of the piston chamber and the piston chamber further comprises a circumferentially spaced series of elastic tension springs connected between the stabilizer and the seal piston.